Update slevomat/coding-standard requirement from ^6.3.8 to ^6.3.9

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@6.3.8...6.3.9)

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

Author: dependabot-preview[bot]

composer.json

@@ -23,7 +23,7 @@
     "require": {
         "php": "^7.2 || ^8.0",
         "dealerdirect/phpcodesniffer-composer-installer": "^0.6.2",
-        "slevomat/coding-standard": "^6.3.8",
+        "slevomat/coding-standard": "^6.3.9",
         "squizlabs/php_codesniffer": "^3.5.5"
     },
     "config": {