Update to OCaml 5.5.0

[WardBrian]

OCaml 5.5.0 was recently released. At the same time, the plan is to sunset 4.14 at the end of the year, so this seemed like a good opportunity to upgrade.

Even though we're jumping a major version and 5 minor versions, there are actually no breaking changes that affect us directly; the only code changes in this PR are to help with a slowdown related to the Marshal functionality that we use for loading in the compiled list of function signatures.

There are a few places I think the compiler code could be improved by using OCaml 5's features, but those are all left for future work.

Draft until

• Add Ocaml 5.5.0 ocaml-cross/opam-cross-windows#407 is merged
• Allow core and dependencies to be installed on arm32 ocaml/opam-repository#30147 is merged

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here:
  • OR, no user-facing changes were made

Release notes

The Stan compiler is now built with OCaml 5.5.0 by default

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.42%. Comparing base (97ef880) to head (27bc284).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1631      +/-   ##
==========================================
- Coverage   90.49%   90.42%   -0.08%     
==========================================
  Files          65       65              
  Lines       10105    10087      -18     
==========================================
- Hits         9145     9121      -24     
- Misses        960      966       +6     

Files with missing lines	Coverage Δ
src/stan_math_signatures/Stan_math_signatures.ml	86.27% <ø> (ø)

... and 14 files with indirect coverage changes

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[WardBrian]

cc @nhuurre @SteveBronder: Next time you want to build the compiler, you will need a new opam switch: cd scripts && bash ./setup_dev_env.sh