I see that there are open source successors to Centos, and AlmaLinux
appears to be a strong contender - CERN chose it, and they know what they are doing. It’s RedHat flavour so your sysadmin skills will transfer.
I’m not confident that you can solve your problem by cherry-picking a Docker. Processes which run within Docker use the same kernel as all others, and I think the kernel version - the actual version, not the patchlevel - will need to be newer than the one you have.
I second the idea that the best approach is to provision a new server with a new OS and restore a Discourse backup.