[TASK] Also allow higher versions of PHPUnit

This hopefully will allow us to run the unit tests for all
PHP version that we claim to support, i.e., from 5.6.x to currently
8.4.x.

Author: oliverklee

composer.json

@@ -27,7 +27,7 @@
         "ext-iconv": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.7.27"
+        "phpunit/phpunit": "^5.7.27 || ^8.5.39"
     },
     "suggest": {
         "ext-mbstring": "for parsing UTF-8 CSS"