Bump phpunit/phpunit from 9.6.22 to 11.5.5

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 9.6.22 to 11.5.5.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/11.5.5/ChangeLog-11.5.md)
- [Commits](sebastianbergmann/phpunit@9.6.22...11.5.5)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

composer.json

@@ -19,7 +19,7 @@
     "require-dev": {
         "pantheon-systems/pantheon-wp-coding-standards": "^2.0",
         "pantheon-systems/wpunit-helpers": "^2.0",
-        "phpunit/phpunit": "^9",
+        "phpunit/phpunit": "^11",
         "yoast/phpunit-polyfills": "^3.0",
         "automattic/vipwpcs": "^3.0"
     },