[ci] Bump actions/cache from 3 to 4

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]>

.github/workflows/ci.yml

@@ -62,7 +62,7 @@ jobs:
 
       - name: Cache OpenWrt dependencies
         id: cache
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         env:
           cache-name: cache-openwisp-config-dependencies
         with: