Add opam files for coq-metacoq version 1.2.1 for Coq 8.17 and 8.18

[yforster]

No description provided.

[palmskog]

@yforster please add the following line at the top of every .opam file:

opam-version: "2.0"

[palmskog]

@yforster CI will easily time out on both 8.17 and 8.18, possibly missing some packages. If I merge this PR after timeout, could you double check locally that both sets of packages install properly?

[yforster]

I added the opam-version line, thanks

[yforster]

Can I double check already before you merge? Or do I need to wait for the packages to appear in the released repo?

[palmskog]

It would be nice if we could get a check after the merge (after locally running opam update), since weird things can go wrong with opam and package constraints...

[yforster]

Locally, running opam repo add test . on a new switch and then installing both coq-metacoq.1.2.1+8.17 and coq-metacoq.1.2.1+8.18 works fine. I'll double check after the merge, but everything should work. Also, I just discovered that https://coq.inria.fr/opam-packaging.html exists and I think that's a great explanation :)