How I got apt update and install to work (with Debian Jessie retired)

Thanks for the advice posted here:

I've done the update, which introduced one additional error which I've highlighted below.

root@mainnas:~# apt-get update
Hit:1 http://archive.debian.org/debian-security jessie/updates InRelease
Ign:2 http://archive.kernel.org/debian-archive/debian jessie InRelease
Get:3 http://archive.kernel.org/debian-archive/debian jessie-backports InRelease [166 kB]
Ign:4 https://apt.readynas.com/packages/readynasos 6.10.9 InRelease
Ign:5 https://apt.readynas.com/packages/readynasos 6.10.9 Release
Get:6 http://archive.kernel.org/debian-archive/debian jessie Release [148 kB]
Ign:3 http://archive.kernel.org/debian-archive/debian jessie-backports InRelease
Ign:7 https://apt.readynas.com/packages/readynasos 6.10.9/updates amd64 Packages.diff/Index
Ign:8 https://apt.readynas.com/packages/readynasos 6.10.9/updates all Packages
Get:9 http://archive.kernel.org/debian-archive/debian jessie Release.gpg [2,420 B]
Ign:9 http://archive.kernel.org/debian-archive/debian jessie Release.gpg
Ign:10 https://apt.readynas.com/packages/readynasos 6.10.9/apps amd64 Packages.diff/Index
Get:11 http://archive.kernel.org/debian-archive/debian jessie/main amd64 Packages [6,818 kB]
Ign:12 https://apt.readynas.com/packages/readynasos 6.10.9/apps all Packages
Ign:13 https://apt.readynas.com/packages/readynasos 6.10.9/main amd64 Packages.diff/Index
Ign:14 https://apt.readynas.com/packages/readynasos 6.10.9/main all Packages
Ign:15 https://apt.readynas.com/packages/readynasos 6.10.9/updates amd64 Packages
Ign:8 https://apt.readynas.com/packages/readynasos 6.10.9/updates all Packages
Ign:16 https://apt.readynas.com/packages/readynasos 6.10.9/apps amd64 Packages
Ign:12 https://apt.readynas.com/packages/readynasos 6.10.9/apps all Packages
Ign:17 https://apt.readynas.com/packages/readynasos 6.10.9/main amd64 Packages
Ign:14 https://apt.readynas.com/packages/readynasos 6.10.9/main all Packages
Ign:15 https://apt.readynas.com/packages/readynasos 6.10.9/updates amd64 Packages
Ign:8 https://apt.readynas.com/packages/readynasos 6.10.9/updates all Packages
Ign:16 https://apt.readynas.com/packages/readynasos 6.10.9/apps amd64 Packages
Ign:12 https://apt.readynas.com/packages/readynasos 6.10.9/apps all Packages
Ign:17 https://apt.readynas.com/packages/readynasos 6.10.9/main amd64 Packages
Ign:14 https://apt.readynas.com/packages/readynasos 6.10.9/main all Packages
Ign:15 https://apt.readynas.com/packages/readynasos 6.10.9/updates amd64 Packages
Ign:8 https://apt.readynas.com/packages/readynasos 6.10.9/updates all Packages
Ign:16 https://apt.readynas.com/packages/readynasos 6.10.9/apps amd64 Packages
Ign:12 https://apt.readynas.com/packages/readynasos 6.10.9/apps all Packages
Ign:17 https://apt.readynas.com/packages/readynasos 6.10.9/main amd64 Packages
Ign:14 https://apt.readynas.com/packages/readynasos 6.10.9/main all Packages
Ign:15 https://apt.readynas.com/packages/readynasos 6.10.9/updates amd64 Packages
Ign:8 https://apt.readynas.com/packages/readynasos 6.10.9/updates all Packages
Ign:16 https://apt.readynas.com/packages/readynasos 6.10.9/apps amd64 Packages
Ign:12 https://apt.readynas.com/packages/readynasos 6.10.9/apps all Packages
Ign:17 https://apt.readynas.com/packages/readynasos 6.10.9/main amd64 Packages
Ign:14 https://apt.readynas.com/packages/readynasos 6.10.9/main all Packages
Ign:15 https://apt.readynas.com/packages/readynasos 6.10.9/updates amd64 Packages
Ign:8 https://apt.readynas.com/packages/readynasos 6.10.9/updates all Packages
Ign:16 https://apt.readynas.com/packages/readynasos 6.10.9/apps amd64 Packages
Ign:12 https://apt.readynas.com/packages/readynasos 6.10.9/apps all Packages
Ign:17 https://apt.readynas.com/packages/readynasos 6.10.9/main amd64 Packages
Ign:14 https://apt.readynas.com/packages/readynasos 6.10.9/main all Packages
Err:15 https://apt.readynas.com/packages/readynasos 6.10.9/updates amd64 Packages
403 Forbidden
Get:18 http://archive.kernel.org/debian-archive/debian jessie/contrib amd64 Packages [49.8 kB]
Get:19 http://archive.kernel.org/debian-archive/debian jessie/non-free amd64 Packages [83.2 kB]
Ign:16 https://apt.readynas.com/packages/readynasos 6.10.9/apps amd64 Packages
Ign:17 https://apt.readynas.com/packages/readynasos 6.10.9/main amd64 Packages
Fetched 7,267 kB in 16s (438 kB/s)
Reading package lists... Done
W: The repository 'https://apt.readynas.com/packages/readynasos 6.10.9 Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://archive.kernel.org/debian-archive/debian jessie-backports InRelease: The following signatures were invalid: KEYEXPIRED 1587841717 KEYEXPIRED 1668891673
W: GPG error: http://archive.kernel.org/debian-archive/debian jessie Release: The following signatures were invalid: KEYEXPIRED 1587841717
E: Failed to fetch https://apt.readynas.com/packages/readynasos/dists/6.10.9/updates/binary-amd64/Packages 403 Forbidden
E: Some index files failed to download. They have been ignored, or old ones used instead.

I am trying to install a plugin for Lyrion music server (which installed successfully after the update), but I am getting an fatal error saying Kernel too old with one of the binaries used for the addon.

If I use the other binary I get other errors:

/var/lib/squeezeboxserver/cache/InstalledPlugins/Plugins/RaopBridge/Bin/squeeze2raop-linux-x86_64: /usr/lib/x86 _64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.21' not found (required by /var/lib/squeezeboxserver/cache/I nstalledPlugins/Plugins/RaopBridge/Bin/squeeze2raop-linux-x86_64)
/var/lib/squeezeboxserver/cache/InstalledPlugins/Plugins/RaopBridge/Bin/squeeze2raop-linux-x86_64: /usr/lib/x86 _64-linux-gnu/libstdc++.so.6: version `CXXABI_1.3.9' not found (required by /var/lib/squeezeboxserver/cache/Ins talledPlugins/Plugins/RaopBridge/Bin/squeeze2raop-linux-x86_64)

If I apt-get install libstdc++6 it is there and a much newer version. 

So I seem to be stuck between the proverbial rock and a hard place.

Any advice welcome, I have of course presented this problem to the owner of the addon as well.