Test CI: Exclude PHP "8+" for the moment

As PHPUnit 7.5 is the latest version compatible with PHP 7.1 but not compatible with PHP 8+

Author: pH-7

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         os: [ 'ubuntu-latest' ]
-        php-versions: [ '7.1', '7.2', '7.3', '7.4', '8.0' ]
+        php-versions: [ '7.1', '7.2', '7.3', '7.4' ]
         phpunit-version: [ '9.5' ]
     name: PHP ${{ matrix.php-versions }} Test on ${{ matrix.os }}
     steps: