Update actions/cache action to v6

Author: renovate[bot]

.github/actions/php-build/action.yml

@@ -26,7 +26,7 @@ runs:
     - name: 'Cache Composer Dependencies'
       id: composer-cache
       if: ${{ inputs.cache-deps == 'true' }}
-      uses: actions/cache@v5
+      uses: actions/cache@v6
       with:
         key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}
         path: './src/vendor'