maintenance: avoid deprecated bindings in uuidm 0.9.9

[psafont]

Unblocks the CI on all open PRs

[edwintorok]

This is not safe for OCaml 5 (in fact I think Uuidm's own documentation is wrong here, and hasn't been updated for OCaml 5). Sharing a State.t between domains is a bad idea.

Although this change doesn't introduce any new problems, because we've already had this problem before, we just didn't see it because it was hidden inside Uuidm.
(in fact strictly speaking Uuidm was only thread-safe previously if you ever called Uuidm on the main thread once to force the lazy value, because we know that on OCaml 4 forcing lazy values from multiple threads can lead to a segfault).

Lets revisit this when we switch to OCaml 5, we can then use Random.split () here, and be careful to avoid partial application).
Or we could use Uuid.make_uuid_fast which solved all these problems already and has added a mutex to make this work on both OCaml 4 and 5.
(if you're guessing how I knew about these problems, it is because I've recently looked at how all this works when making changs to our uuidx module).

[edwintorok]

This doesn't appear to introduce any new bugs though (the domain-safety issue was also there with the old code).