Bump actions/cache from 4 to 6 (#39081)

Bumps [actions/cache](https://github.com/actions/cache) from 4 to 6.
- [Release notes](https://github.com/actions/cache/releases)
- [Changelog](https://github.com/actions/cache/blob/main/RELEASES.md)
- [Commits](actions/cache@v4...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>

Author: dependabot[bot]

.github/workflows/playground_frontend_test.yml

@@ -48,7 +48,7 @@ jobs:
       - uses: actions/checkout@v7
 
       - name: 'Cache Flutter Dependencies'
-        uses: actions/cache@v4
+        uses: actions/cache@v6
         with:
           path: /opt/hostedtoolcache/flutter
           key: ${{ runner.OS }}-flutter-install-cache-${{ env.FLUTTER_VERSION }}

.github/workflows/tour_of_beam_frontend_test.yml

@@ -50,7 +50,7 @@ jobs:
       - uses: actions/checkout@v7
 
       - name: 'Cache Flutter Dependencies'
-        uses: actions/cache@v4
+        uses: actions/cache@v6
         with:
           path: /opt/hostedtoolcache/flutter
           key: ${{ runner.OS }}-flutter-install-cache-${{ env.FLUTTER_VERSION }}