Check that pre-releases have the avoid-version flag

[kit-ty-kate]

See ocaml/opam-repository#26799

Most pre-releases should have at least:

flags: avoid-version
available: opam-version >= "2.1.0"

There are of course exceptions (e.g. ocaml-base-compiler which uses the ocaml-beta trick instead) but in general this is true, so it would make sense to have a warning in the linter for this

[shonfeder]

This should be an easy lint to add, but in light of the discussion in ocaml/infrastructure#152 (comment) I'd like to be sure we validate the lint check through whatever explicit process we settle on for these changes.

I'll put this on the queue for the opam maintenance meeting on Thurs.

[punchagan]

I'll put this on the queue for the opam maintenance meeting on Thurs.

@shonfeder curious to know if this did make it to the opam maintenance discussion on Thursday.

[shonfeder]

It did not! We had too many things ahead it in the queue. I'll report back after the meeting tomorrow.