Update wp-phpunit/wp-phpunit requirement from ^6.5 to ^6.6

Updates the requirements on [wp-phpunit/wp-phpunit](https://github.com/wp-phpunit/wp-phpunit) to permit the latest version.
- [Commits](wp-phpunit/wp-phpunit@6.5.0...6.6.0)

---
updated-dependencies:
- dependency-name: wp-phpunit/wp-phpunit
  dependency-type: direct:development
...

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

composer.json

@@ -38,7 +38,7 @@
 		"rector/rector": "0.17.10",
 		"roots/wordpress": "^6.5",
 		"syntatis/coding-standard": "^1.3",
-		"wp-phpunit/wp-phpunit": "^6.5",
+		"wp-phpunit/wp-phpunit": "^6.6",
 		"yoast/phpunit-polyfills": "^2.0"
 	},
 	"scripts": {