Docs: Recommend that packages using non-public Base functionality specify a tilde or equals compat entry for Julia

[DilumAluthge]

This is a follow-up to #35715.

Motivation

As I see it, my reasoning here is similar to our reasoning for requiring upper-bounded [compat] entries for packages registered in General. We don't let people register a package with a [compat] entry of the form MyDep = ">= 1.1.1", because there's no way for the package author to know or guarantee that MyDep v2 won't break their package.

Similarly, if a package author is using non-public Base functionality, then I don't think they should have a [compat] entry of the form julia = "1.12" (or julia = "^1.12"), because there's no way for the package author to know whether or not Julia 1.13 will break (or remove) the non-public functionality. It's thus safer for the package author to do julia = "~1.12", which will prevent users of Julia 1.13 from installing a broken version of the package.

A very risk-averse package author would do julia = "=1.12.0, =1.12.1, =1.12.2, ...", in case the functionality they rely on changes in a patch version. I don't think we need to recommend that level of strictness.

Arguments against tilde (or equals) [compat] specifiers

Breaks PkgEval

One argument against julia = "~1.12" is that it renders the package untestable in the PkgEval for the release-1.13 branch, because the Julia version on the release-1.13 branch is of the form 1.13.0-DEV..., which is incompatible with julia = "~1.12". So PkgEval wouldn't be able to test any packages with tilde (or equals) Julia [compat] entries. This is a problem, because the same package that uses non-Base public functionality is probably using some public Base functionality, and we want to run PkgEval to make sure we don't break the public Base functionality in that package.

I think we can work around this by having some special logic in PkgEval that rewrites tilde/equals compat entries to caret compat entries, thus allowing us to test these packages in PkgEval.