GA: Remove PHP 8.1 from the matrix

Looks like PHPUnit v7 does not work with it, and we can't upgrade PHPUnit version
keeping compatibility with older PHP and PHPUnit versions.

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        php: [5.5, 5.6, 7.0, 7.1, 7.2, 7.3, 7.4, 8.0, 8.1]
+        php: [5.5, 5.6, 7.0, 7.1, 7.2, 7.3, 7.4, 8.0]
 
     name: PHP ${{ matrix.php }}/Linux