Revert "Test on PHP 8.1 too (#467)"

This reverts commit 692830c.

Author: GrahamCampbell

.github/workflows/tests.yml

@@ -11,7 +11,7 @@ jobs:
 
     strategy:
       matrix:
-        php: ['5.3', '5.4', '5.5', '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1']
+        php: ['5.3', '5.4', '5.5', '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0']
 
     steps:
       - name: Checkout Code