Update actions/cache action to v6

Author: renovate[bot]

.github/workflows/backend.yml

@@ -42,7 +42,7 @@ jobs:
           key: ${{ env.PHP_EXTENSIONS_CACHE_KEY }}
 
       - name: Cache php extensions
-        uses: actions/cache@cdf6c1fa76f9f475f3d7449005a359c84ca0f306 # v5
+        uses: actions/cache@2c8a9bd7457de244a408f35966fab2fb45fda9c8 # v6
         with:
           path: ${{ steps.extcache.outputs.dir }}
           key: ${{ steps.extcache.outputs.key }}
@@ -70,7 +70,7 @@ jobs:
         run: echo "::set-output name=dir::$(composer config cache-files-dir)"
 
       - name: Cache dependencies
-        uses: actions/cache@cdf6c1fa76f9f475f3d7449005a359c84ca0f306 # v5
+        uses: actions/cache@2c8a9bd7457de244a408f35966fab2fb45fda9c8 # v6
         with:
           path: ${{ steps.composer-cache.outputs.dir }}
           key: ${{ runner.os }}-${{ hashFiles('**/composer.lock') }}-composer