ci: bump actions/cache from 3 to 4 (#37)

Bumps [actions/cache](https://github.com/actions/cache) from 3 to 4.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](actions/cache@v3...v4)

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

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

.github/workflows/tests.yml

@@ -21,7 +21,7 @@ jobs:
 
       - name: Cache Homebrew Bundler RubyGems
         id: cache
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: ${{ steps.set-up-homebrew.outputs.gems-path }}
           key: ${{ runner.os }}-rubygems-${{ steps.set-up-homebrew.outputs.gems-hash }}