Update phpunit/phpunit requirement from ^9.6.14 to ^9.6.15 (#1038)

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](https://github.com/sebastianbergmann/phpunit/compare/9.6.14...9.6.15)

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

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

composer.json

@@ -56,7 +56,7 @@
 		"phpstan/phpstan": "^1.10.47",
 		"phpstan/phpstan-phpunit": "^1.3.15",
 		"phpstan/phpstan-strict-rules": "^1.5.2",
-		"phpunit/phpunit": "^9.6.14",
+		"phpunit/phpunit": "^9.6.15",
 		"saschaegerer/phpstan-typo3": "^1.9.1",
 		"seld/jsonlint": "^1.10.0",
 		"squizlabs/php_codesniffer": "^3.7.2",