Update slevomat/coding-standard requirement from ^5.0 to ^6.0

Updates the requirements on [slevomat/coding-standard](https://github.com/slevomat/coding-standard) to permit the latest version.
- [Release notes](https://github.com/slevomat/coding-standard/releases)
- [Commits](slevomat/coding-standard@5.0.0...6.0.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

composer.json

@@ -26,7 +26,7 @@
     "require-dev": {
         "phpstan/phpstan": "^0.12-dev",
         "phpunit/phpunit": "^8.0",
-        "slevomat/coding-standard": "^5.0"
+        "slevomat/coding-standard": "^6.0"
     },
     "autoload": {
         "psr-4": {