How do I update Debian Jessie repository sources?

OK, I've seen other posts about issues with installing apps.  What I've seen hasn't helped me.  Here is what I see when I do an apt update:

root@Pro2A:~# apt update
Hit:1 http://security.debian.org jessie/updates InRelease
Ign:2 http://mirrors.edge.kernel.org/debian jessie InRelease
Hit:3 http://mirrors.edge.kernel.org/debian jessie Release
Hit:4 https://download.docker.com/linux/debian jessie InRelease
Err:5 http://mirrors.kernel.org/debian jessie Release.gpg
  The following signatures were invalid: KEYEXPIRED 1668891673
Hit:6 https://apt.readynas.com/packages/readynasos 6.10.8 InRelease
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://mirrors.edge.kernel.org/debian jessie Release: The following signatures were invalid: KEYEXPIRED 1668891673

etc/apt/sources.list contains this:

deb https://apt.readynas.com/packages/readynasos 6.10.8 updates apps main 

deb http://mirrors.kernel.org/debian jessie main
deb http://security.debian.org/ jessie/updates main

6.10.8 is replaced with 6.9.6 on units running the long-term stable release.  I also get some stuff about ZeroTier on units with it installed, but it's not on this NAS and isn't common to all.

Based on something I read elsewhere in the forum with regard to Docker, I tried  apt-key adv --keyserver apt.readynas.com --recv-keys, and that seemed to do something:

Executing: /tmp/tmp.OCrjYmplqk/gpg.1.sh --keyserver
apt.readynas.com

But in the end, I get the same error when doing an apt update.