Set phpunit version from ^8.0 to ^8

Ref: #261
Ref: https://phpunit.de/getting-started/phpunit-8.html

Signed-off-by: William Desportes <williamdes@wdes.fr>

Author: williamdes

composer.json

@@ -22,7 +22,7 @@
     "require-dev": {
         "sami/sami": "^4.0",
         "phpunit/php-code-coverage": "*",
-        "phpunit/phpunit": "^7.4 || ^8.0",
+        "phpunit/phpunit": "^7.4 || ^8",
         "phpmyadmin/coding-standard": "^1.0"
     },
     "conflict": {