Update OCaml to 5.4

[andy1li]

Note

Updates the OCaml toolchain to 5.4 and aligns build tooling.

• Switches buildpack to ocaml-5.4 in codecrafters.yml across compiled starters and solution templates
• Bumps dune to 3.21: (lang dune 3.21) in dune-project and "dune" {>= "3.21"} in .opam files; adds x-maintenance-intent: ["(latest)"]
• Adds dockerfiles/ocaml-5.4.Dockerfile using ocaml/opam:debian-13-ocaml-5.4, installs dune.3.21.0, sets PATH, and caches dependencies

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