GHA: Add OCaml 5.3 to the build matrix

[kit-ty-kate]

Split off from https://github.com/ocaml/opam/pull/6189 as OCaml 5.3 is not released yet and contains several issues such as incompatibility with C++ (https://github.com/ocaml/ocaml/issues/13422) and Cygwin (https://github.com/ocaml/ocaml/issues/13421)

[dra27]

Why are we dropping i686 builds completely? I can't remember - do the Linux builds use opam to create the compiler switches, or did we optimise it with a system compiler?

[kit-ty-kate]

@dra27 @MisterDA do you have an idea what's wrong with the cygwin port with 5.3.0?

  ** Cannot resolve symbols for descriptor object:
   _ZTH10caml_state
  File "caml_startup", line 1:
  Error: Error during linking (exit code 2)

[kit-ty-kate]

https://github.com/ocaml/ocaml/pull/13591 was supposed to fix this issue (https://github.com/ocaml/ocaml/issues/13541) but it doesn't look like it did

[kit-ty-kate]

cc @NickBarnes as well just in case since you also reviewed that PR

[kit-ty-kate]

C++ support is indeed broken on Cygwin (see mention in https://github.com/MisterDA/ocaml/pull/118). I've disabled it temporarily while this is being debugged and fixed upstream (hopefully in a future 5.3.1 release).

In the meantime this is ready to review and merge