Bump actions/cache from 5 to 6 (#14516)

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/build_all.yml

@@ -223,14 +223,14 @@ jobs:
     # Manage flatpak-builder cache externally so PRs restore but never upload
     - name: Restore flatpak-builder cache
       if: github.event_name == 'pull_request'
-      uses: actions/cache/restore@v5
+      uses: actions/cache/restore@v6
       with:
         path: .flatpak-builder
         key: flatpak-builder-${{ matrix.variant.arch }}-${{ github.event.pull_request.base.sha }}
         restore-keys: flatpak-builder-${{ matrix.variant.arch }}-
     - name: Save/restore flatpak-builder cache
       if: github.event_name != 'pull_request'
-      uses: actions/cache@v5
+      uses: actions/cache@v6
       with:
         path: .flatpak-builder
         key: flatpak-builder-${{ matrix.variant.arch }}-${{ github.sha }}