Update phpro/grumphp requirement from ^0.17.1 || ^0.19.0 to ^0.17.1 || ^0.19.0 || ^0.21.0

Updates the requirements on phpro/grumphp to permit the latest version.

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

composer.json

@@ -17,7 +17,7 @@
         "psr/event-dispatcher": "^1.0"
     },
     "require-dev": {
-        "phpro/grumphp": "^0.17.1 || ^0.19.0",
+        "phpro/grumphp": "^0.17.1 || ^0.19.0 || ^0.21.0",
         "phpstan/phpstan": "^0.11.5 || ^0.12.0",
         "phpunit/phpunit": "^8.0 || ^9.0",
         "squizlabs/php_codesniffer": "^3.4",