Bump actions/cache from 4 to 5 (#8482)

Bumps [actions/cache](https://github.com/actions/cache) from 4 to 5.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](https://github.com/actions/cache/compare/v4...v5)

---
updated-dependencies:
- dependency-name: actions/cache
  dependency-version: '5'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/ocaml.yaml

@@ -21,7 +21,7 @@ jobs:
 
       # Cache ccache (shared across runs)
       - name: Cache ccache
-        uses: actions/cache@v5.0.2
+        uses: actions/cache@v5
         with:
           path: ~/.ccache
           key: ${{ runner.os }}-ccache-${{ github.sha }}
@@ -30,7 +30,7 @@ jobs:
 
       # Cache opam (compiler + packages)
       - name: Cache opam
-        uses: actions/cache@v5.0.2
+        uses: actions/cache@v5
         with:
           path: ~/.opam
           key: ${{ runner.os }}-opam-${{ matrix.ocaml-version }}-${{ github.sha }}