build(deps-dev): bump phpunit/phpunit from 9.6.15 to 9.6.21

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 9.6.15 to 9.6.21.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/9.6.21/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@9.6.15...9.6.21)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

composer.json

@@ -40,7 +40,7 @@
 		]
 	},
 	"require-dev": {
-		"phpunit/phpunit": "^9.5",
+		"phpunit/phpunit": "^9.6",
 		"nextcloud/coding-standard": "^1.0"
 	}
 }