Codify policy for removing end-of-life platforms

[shonfeder]

We want to remove Debian 10, which is end of life, from our build matrix.x

We should also have a policy for monitoring other OSs and removing the versions from our matrix when they fall out of LTS.

Perhaps our policy can be recorded in a format like this:

OS	Criteria for removal	Source of truth
Debian	Past LTS EOL	https://wiki.debian.org/DebianReleases

The processes for removing EOL platforms is by deprecating them in ocaml-dockerfile, e.g., ocurrent/ocaml-dockerfile#210

[punchagan]

https://endoflife.date/ might be helpful for tracking EoLs.

[shonfeder]

Yes, that's a good idea!

The policy should include reference to the supported tiers. I think we can take as the source of truth for that https://github.com/ocaml/ocaml/blob/trunk/README.adoc#overview