Updated actions/cache to v4

Author: tabuna

.github/workflows/phpunit.yaml

@@ -42,7 +42,7 @@ jobs:
         id: composer-cache
         run: echo "::set-output name=dir::$(composer config cache-files-dir)"
       - name: Cache composer dependencies
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: ${{ steps.composer-cache.outputs.dir }}
           # Use composer.json for key, if composer.lock is not committed.