Update phpunit/phpunit requirement from ^9.6.18 to ^9.6.19

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/9.6.19/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@9.6.18...9.6.19)

---
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": "6.5.2",
  "yoast/phpunit-polyfills": "^1.0.5",
- "phpunit/phpunit": "^9.6.18",
+ "phpunit/phpunit": "^9.6.19",
  "lucatume/wp-browser": "~3.2.3",
  "codeception/module-asserts": "~1.3.1",
  "codeception/module-phpbrowser": "~1.0.3",