Bump code coverage workflow to PHP 8.1

Author: Sebastian Krätzig

.github/workflows/php_code_coverage.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         operating-system: ['ubuntu-20.04']
-        php-versions: ['8.0']
+        php-versions: ['8.1']
 
     steps:
     - name: Setup PHP