Master branch compiles on Coq 8.15

[proux01]

Update OPAM files and Docker CI accordingly

[gares]

you need to version the .out file, an old version of elpi was giving the character at the end of a text chunk, rather than at the beginning...

[proux01]

@gares thanks, this now works on 8.15, however the CI still shows a few failures:

• some failures in mcHB are due to missing overlays, I could copy/paste them from Porting to HB  math-comp#733 but not sure it's worth it
• failures of analysis and deriving in mcHB are expected
• planB: seems currently broken, do we want to maintain it?

So I would lean toward merging this and possibly fix planB in a separate PR.

[gares]

I'm OK with your plan.
I think planB still makes some sense to "learn" what HB does, so I won't remove it straight away, but I did not look into the error