Bump phpunit/phpunit from 9.6.22 to 12.3.6

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 9.6.22 to 12.3.6.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/12.3.6/ChangeLog-12.3.md)
- [Commits](sebastianbergmann/phpunit@9.6.22...12.3.6)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-version: 12.3.6
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

composer.json

@@ -14,7 +14,7 @@
   "require-dev": {
       "pantheon-systems/pantheon-wp-coding-standards": "^2.0",
       "pantheon-systems/pantheon-wordpress-upstream-tests": "dev-master",
-      "phpunit/phpunit": "^9",
+      "phpunit/phpunit": "^12",
       "yoast/phpunit-polyfills": "^4.0",
     "pantheon-systems/wpunit-helpers": "^2.0"
   },