automate opam release

[gares]

maybe it is time to turn

https://github.com/math-comp/hierarchy-builder/blob/3d90825f49338bba4729dff03b0f927f760843ab/.github/workflows/main.yml#L35-L89

into a GH action and just apply it here...

[pi8027]

I would rather discontinue Algebra Tactics in its current form (and remove it from Rocq Platform) since @proux01 is porting it to the main MathComp repo. Then, I may want to reuse this repo to experiment with my new ideas, but I won't make a release for a while, so that no one will depend on it.

[proux01]

fine by me (that being said, the auto opam release is pretty nice, we should probably use it in mathcomp itself)