Update slevomat/coding-standard requirement from 8.7.1 to 8.8.0 (#206)

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@8.7.1...8.8.0)

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

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composer.json

@@ -15,7 +15,7 @@
     "php": "^7.2 || ^8.0",
     "phpcompatibility/phpcompatibility-wp": "2.1.4",
     "sirbrillig/phpcs-variable-analysis": "2.11.9",
-    "slevomat/coding-standard": "8.7.1",
+    "slevomat/coding-standard": "8.8.0",
     "wp-coding-standards/wpcs": "2.3.0"
   },
   "suggest": {