github-actions(deps): bump actions/cache from 5 to 6

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](actions/cache@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>

Author: dependabot[bot]

.github/workflows/diagnostics.yml

@@ -42,7 +42,7 @@ jobs:
           php-version: ${{ matrix.php }}
 
       - name: Cache composer cache directory
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         env:
           cache-name: composer-cache-dir-${{ matrix.php }}
         with:
@@ -53,7 +53,7 @@ jobs:
         run: composer install
 
       - name: Cache vendor directory
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         env:
           cache-name: vendor-${{ matrix.php }}
         with:
@@ -66,7 +66,7 @@ jobs:
         run: ./vendor/bin/phpcq ${{ matrix.phpcq_install }} -v
 
       - name: Cache phpcq directory
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         env:
           cache-name: phpcq-${{ matrix.php }}
         with: