Revert to phpunit ^5.7.0 and limit test matrix to PHP 5.6-7.4

Author: ngekoding

.github/workflows/ci.yml

@@ -17,7 +17,6 @@ jobs:
           - '7.2'
           - '7.3'
           - '7.4'
-          - '8.0'
 
     steps:
       - uses: actions/checkout@v4

composer.json

@@ -20,6 +20,6 @@
         "php": ">=5.6"
     },
     "require-dev": {
-        "phpunit/phpunit": "*"
+        "phpunit/phpunit": "^5.7.0"
     }
 }