Bump actions/cache from 2 to 4

Bumps [actions/cache](https://github.com/actions/cache) from 2 to 4.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](actions/cache@v2...v4)

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

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/CI.yml

@@ -21,7 +21,7 @@ jobs:
           docs-folder: "."
           build-command: "make dirhtml"
       - name: Cache artifacts
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         if: github.ref == 'refs/heads/main'
         with:
           key: html-${{ github.sha }}
@@ -34,7 +34,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Cache artifacts
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           key: html-${{ github.sha }}
           path: ./_build/dirhtml