Upgrade to opam 2.2

[kit-ty-kate]

Queued on #379

[Leonidas-from-XIV]

I assume that requires the OPAM dependencies to be bumped to 2.2?

[kit-ty-kate]

yes but that won't work until 2.2.0~alpha1 is released

[Leonidas-from-XIV]

Merged #379, so let's wait until the alpha.

[psafont]

I'm unable to use opam monorepo lock with 2.2.0~beta2 on github.com/mirage/mirage-xen because it doesn't detect the presence of the dune-universe repository, does this PR fix it?

$ opam repo
[NOTE] These are the repositories in use by the current switch. Use '--all' to see all configured repositories.

<><> Repository configuration for switch mirage <><><><><><><><><><><><><><><><>
 1 dune-universe git+https://github.com/dune-universe/opam-overlays.git
 2 default       https://opam.ocaml.org

$ opam monorepo lock
==> Using 1 locally scanned package as the target.
opam-monorepo: [ERROR] Some dependencies cannot be built with dune!

opam-monorepo requires that all dependencies use dune as their build system.

These dependencies (possibly transitive) don't use dune as their build system:
- ocamlfind
- logs
- fmt
- cmdliner

The dune-universe opam repository (git+https://github.com/dune-universe/opam-overlays.git) contains dune ports of some popular packages to help build more packages with dune. It doesn't appear to be set up on this switch. Adding it to this switch may fix this issue. Add the dune-universe opam repository to this switch by running the command:

opam repository add dune-universe git+https://github.com/dune-universe/opam-overlays.git