new version of coq-mtac2 to 2.1.4+9.1

[Justme0606]

No description provided.

[silene]

This fails to compile:

CAMLOPT -c -for-pack MetaCoqPlugin src/metaCoqInit.ml
File "src/metaCoqInit.mlg", line 122, characters 3-12:
Error: Unbound constructor "ProofMode"

I think the ProofMode constructor was introduced in Rocq 9.1, which means that there should be a constraint "coq-core" (>= "9.1") instead of "coq" (>= "9.0"). (And it would make sense, given the name of the tarball.)

But note that Unicoq, which is a dependency, is currently marked as incompatible with Rocq 9.1. So, either a new release of Unicoq is needed, or the current one needs some relaxed bounds.