[TASK] Update phpunit/phpunit requirement from ^9.6.19 to ^9.6.20 (#1364)

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/9.6.20/ChangeLog-9.6.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/compare/9.6.19...9.6.20)

---
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

@@ -55,7 +55,7 @@
 		"phpstan/phpstan": "^1.11.7",
 		"phpstan/phpstan-phpunit": "^1.4.0",
 		"phpstan/phpstan-strict-rules": "^1.6.0",
-		"phpunit/phpunit": "^9.6.19",
+		"phpunit/phpunit": "^9.6.20",
 		"saschaegerer/phpstan-typo3": "^1.10.1",
 		"seld/jsonlint": "^1.10.2",
 		"spaze/phpstan-disallowed-calls": "^3.4",