Update slevomat/coding-standard requirement from ~6.0 to ~8.13

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@6.0.0...8.13.1)

---
updated-dependencies:
- dependency-name: slevomat/coding-standard
  dependency-type: direct:development
...

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

composer.json

@@ -40,7 +40,7 @@
     "phpunit/phpunit": "^9",
     "roave/security-advisories": "dev-master",
     "roots/wordpress": "^6.0.0",
-    "slevomat/coding-standard": "~6.0",
+    "slevomat/coding-standard": "~8.13",
     "squizlabs/php_codesniffer": "^3.2",
     "wp-phpunit/wp-phpunit": "^6.0.0",
     "yoast/phpunit-polyfills": "^1.0.5 || ^2.0"