test: add php81

Author: cesargb

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
         strategy:
             fail-fast: false
             matrix:
-                php: [7.4, 8.0]
+                php: [7.4, 8.0, 8.1]
                 laravel: [8.*, 6.*]
                 dependency-version: [prefer-stable]
                 include: