Update sponsors in README

Bumps [actions/cache](https://github.com/actions/cache) from 5 to 6.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](https://github.com/actions/cache/compare/v5...v6)

---
updated-dependencies:
- dependency-name: actions/cache
  dependency-version: '6'
  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/docs.yaml

@@ -81,7 +81,7 @@ jobs:
         run: "cd docs && [[ -f package-lock.json || -f npm-shrinkwrap.json ]] && npm ci || true"
       - name: Cache Restore
         id: cache-restore
-        uses: actions/cache/restore@v5
+        uses: actions/cache/restore@v6
         with:
           path: |
             ${{ runner.temp }}/hugo_cache
@@ -97,7 +97,7 @@ jobs:
             --cacheDir "${{ runner.temp }}/hugo_cache"
       - name: Cache Save
         id: cache-save
-        uses: actions/cache/save@v5
+        uses: actions/cache/save@v6
         with:
           path: |
             ${{ runner.temp }}/hugo_cache