Update slevomat/coding-standard requirement from 8.6.4 to 8.7.1 (#205)

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@8.6.4...8.7.1)

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

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

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": "8.6.4",
+    "slevomat/coding-standard": "8.7.1",
     "wp-coding-standards/wpcs": "2.3.0"
   },
   "suggest": {