Fix CI compiler matrix

[jeet-dekivadia]

Summary

• replace the obsolete OCaml 5.0 alpha and 4.x matrix with the supported OCaml 5 range
• retain Linux, macOS, and Windows coverage while allowing setup-ocaml to select the latest compatible OCaml 5 compiler
• update checkout and setup-ocaml to their current major versions and remove the retired cache-prefix input

Context

base.opam now requires OCaml >= 5.1.0, while the existing matrix still includes an OCaml 5.0 prerelease and OCaml 4.x compilers. The scheduled workflow currently stops during action setup before reaching dependency installation.

With this update, fork CI initializes OCaml 5.4.1 and reaches opam install . --deps-only --with-test on Linux, macOS, and Windows. Current master still depends on unpublished packages such as base_internalhash_types, so the public workflow cannot complete dependency resolution until those packages are published.

Addresses #192.

Validation

• git diff --check
• parsed .github/workflows/workflow.yml with Ruby YAML
• fork GitHub Actions run reached dependency installation on Linux, macOS, and Windows