[PHP]: Update phpunit/phpunit requirement in /vendor-bin/phpunit

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](sebastianbergmann/phpunit@9.6.14...9.6.15)

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

Signed-off-by: dependabot[bot] <support@github.com>

vendor-bin/phpunit/composer.json

@@ -1,7 +1,7 @@
 {
     "require-dev": {
         "dg/bypass-finals": "^1.5",
-        "phpunit/phpunit": "9.6.14",
+        "phpunit/phpunit": "9.6.15",
         "symfony/phpunit-bridge": "7.0.0",
         "vimeo/psalm": "5.16.0",
         "psalm/plugin-phpunit": "0.18.4",