change actions/cache@v2 to v3 due to deprecation

Author: k0ka

.github/workflows/unit_tests.yml

@@ -34,7 +34,7 @@ jobs:
         run: |
           echo "::set-output name=dir::$(composer config cache-files-dir)"
 
-      - uses: actions/cache@v2
+      - uses: actions/cache@v3
         with:
           path: |
             ~/.php_cs.cache