Align finmap with mathcomp releases?

[hoheinzollern]

I just realized that there was some confusion in my PR to release the opam file for distribution, partly due to finmap being just slightly out of sync with mathcomp.

Right now finmap 2.1 is compatible with mathcomp <= 2.3 and finmap 2.2 is compatible with mathcomp 2.4. It would make sense to skip a few release numbers and release finmap 2.4 compatible with mathcomp 2.4, or 2.5 along with the next release of finmap.

Keep in mind that the current finmap 2.2 release does not have the changelog updated so a flush is needed (see #126), and that finmap 2.1 is incompatible with mathcomp 2.4 (see unmerged PR https://github.com/rocq-prover/opam/pull/3391).