Add PHP 8.1 to GitHub workflow strategy

Author: bensherred

.github/workflows/run-tests.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        php: [ 8.0 ]
+        php: [ 8.0, 8.1 ]
         laravel: [ ^8.0 ]
 
     name: P${{ matrix.php }} - L${{ matrix.laravel }}