Update eff.opam for OCaml 5.2

[ncihnegn]

No description provided.

[matijapretnar]

Hmmm, I think the GitHub actions need to be updated as well. Also, I prefer to have the OCamlformat version fixed, otherwise the testing is unpredictable. Can you recommend a version that works on OCaml 5, but is still supported on some version of OCaml 4?

[ncihnegn]

I don't see why CI will fail. Could you please retry it?

[matijapretnar]

I think the problem is that this line in GitHub script installs OCaml 4:

eff/.github/workflows/test.yml

Line 14 in 4c47fe0

uses: avsm/setup-ocaml@v1

[matijapretnar]

The tests now fail with

dune build @fmt --auto-promote
File "dune-project", line 22, characters 2-15:
22 |   (ocamlformat)))
       ^^^^^^^^^^^^^
Error: Atom or quoted string expected
make: *** [Makefile:6: format] Error 1

I think it would be best to keep a fixed version of OCamlformat, preferably one that works with both OCaml 4 and 5, if possible.