update composer dependency to PHP ^8.1 #8

Author: gregor-j

composer.json

@@ -25,7 +25,7 @@
     ],
     "minimum-stability": "stable",
     "require": {
-        "php": ">=7.4"
+        "php": "^8.1"
     },
     "require-dev": {
         "phpunit/phpunit": "^9.6",