composer(deps): bump phpunit/phpunit from 9.5.9 to 9.6.8 in /src/php

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 9.5.9 to 9.6.8.
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/9.6.8/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@9.5.9...9.6.8)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

src/php/composer.json

@@ -3,7 +3,7 @@
     "description": "",
     "require": {
         "php": "^8.0",
-        "phpunit/phpunit": "^9.5"
+        "phpunit/phpunit": "^9.6"
     },
     "autoload-dev": {
         "files": [