Update run_tests.yml

Author: meinder-a

.github/workflows/run_tests.yml

@@ -14,7 +14,7 @@ jobs:
 
     strategy:
       matrix:
-        php: [7.4, 8.0]
+        php: [7.4, 8.0, 8.1]
 
     steps:
       - uses: actions/checkout@v1