Use actions/cache@v4

Author: jamieburchell

.github/workflows/test-phpunit.yml

@@ -152,7 +152,7 @@ jobs:
         id: composer-cache
         run: echo "::set-output name=dir::$(composer config cache-files-dir)"
       - name: Cache composer dependencies
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: ${{ steps.composer-cache.outputs.dir }}
           key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}