Update phpunit/phpunit requirement from 9.6.18 to 9.6.19 (#142)

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:development
...

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

composer.json

@@ -9,7 +9,7 @@
   "require-dev": {
     "ext-mbstring": "*",
     "phpspec/phpspec": "7.5.0",
-    "phpunit/phpunit": "9.6.18",
+    "phpunit/phpunit": "9.6.19",
     "friends-of-phpspec/phpspec-code-coverage": "6.3.0",
     "phpstan/phpstan": "1.10.66",
     "phpstan/extension-installer": "1.3.1",