Add PHP 8.1 to CI

Author: derrabus

Diff for: .github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
         strategy:
             fail-fast: false
             matrix:
-                php: ['7.1.3', '7.2', '7.3', '7.4', '8.0']
+                php: ['7.1.3', '7.2', '7.3', '7.4', '8.0', '8.1']
                 include:
                     - php: '7.4'
                       deps: lowest