php.yml: Test on PHP 8.x

Author: reedy

.github/workflows/php.yml

@@ -16,6 +16,11 @@ jobs:
           - '7.2'
           - '7.3'
           - '7.4'
+          - '8.0'
+          - '8.1'
+          - '8.2'
+          - '8.3'
+          - '8.4'
 
     steps:
     - uses: actions/checkout@v2
@@ -31,7 +36,7 @@ jobs:
       run: composer validate
 
     - name: Cache Composer cache
-      uses: actions/cache@v4
+      uses: actions/cache@v2
       with:
         path: ~/.composer/cache
         key: composer-cache-php${{ matrix.php }}