SP-971 Lock PHPUnit to 10.5.20

Author: bobbrodie

.github/workflows/test.yml

@@ -21,7 +21,7 @@ jobs:
           configuration: phpunit.xml
           php_version: ${{ matrix.php-version }}
           php_extensions: bcmath gmp xdebug
-          version: 10
+          version: 10.5.20
         env:
           XDEBUG_MODE: coverage
   phpcs: