run test also on PHP 8.3

Author: pfefferle

.github/workflows/phpunit.yml

@@ -15,7 +15,7 @@ jobs:
        options: --health-cmd="mysqladmin ping" --health-interval=10s --health-timeout=10s --health-retries=10
     strategy:
       matrix:
-        php-versions: ['5.6', '7.0', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2']
+        php-versions: ['5.6', '7.0', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3']
         include:
           - wp-version: latest
           - wp-version: '6.2'