Update phpunit/phpunit requirement from ^9.6.13 to ^9.6.15

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/9.6.15/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@9.6.13...9.6.15)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:production
...

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

composer.json

@@ -21,7 +21,7 @@
 		"altis/dev-tools-command": "^0.7.2",
 		"wp-phpunit/wp-phpunit": "6.1.3",
 		"yoast/phpunit-polyfills": "^1.0.5",
-		"phpunit/phpunit": "^9.6.13",
+		"phpunit/phpunit": "^9.6.15",
 		"lucatume/wp-browser": "~3.1.10",
 		"codeception/module-asserts": "~1.3.1",
 		"codeception/module-phpbrowser": "~1.0.3",