Update phpunit/phpunit requirement in /build/integration

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/master/ChangeLog-9.5.md)
- [Commits](sebastianbergmann/phpunit@6.5.0...9.5.10)

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

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

build/integration/composer.json

@@ -1,6 +1,6 @@
 {
   "require-dev": {
-    "phpunit/phpunit": "~6.5",
+    "phpunit/phpunit": "~9.5",
     "behat/behat": "~3.7.0",
     "guzzlehttp/guzzle": "6.5.2",
     "jarnaiz/behat-junit-formatter": "^1.3",