Bump actions/cache from 5 to 6 (#15)

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -59,7 +59,7 @@ jobs:
         run: echo "::set-output name=dir::$(composer config cache-dir)"
 
       - name: Cache dependencies
-        uses: actions/cache@v5
+        uses: actions/cache@v6
         with:
           path: ${{ steps.composer-cache.outputs.dir }}
           key: composer-${{ runner.os }}-${{ matrix.php-version }}-${{ hashFiles('**/composer.*') }}-${{ matrix.composer-flags }}