test(php): Add PHP 8.1 for testing (#1256)

Author: klausi

.github/workflows/testing.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php-versions: ['7.3', '7.4', '8.0']
+        php-versions: ['7.3', '7.4', '8.0', '8.1']
         drupal-core: ['9.3.x']
         include:
           # Extra run to also test on latest Drupal 8 and PHP 7.2.