Update slevomat/coding-standard requirement from 6.0.8 to 6.4.0

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@6.0.8...6.4.0)

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

Author: dependabot-preview[bot]

composer.json

@@ -37,7 +37,7 @@
         "orchestra/testbench": "^5.0 || ^6.0",
         "phpmd/phpmd": "^2.6",
         "phpunit/phpunit": "9.*",
-        "slevomat/coding-standard": "6.0.8",
+        "slevomat/coding-standard": "6.4.0",
         "squizlabs/php_codesniffer": "^3.5"
     },
     "autoload": {