Update phpunit/phpunit requirement from ^12.1.5 to ^12.1.6 (#194)

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/12.1.6/ChangeLog-12.1.md)
- [Commits](sebastianbergmann/phpunit@12.1.5...12.1.6)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-version: 12.1.6
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

composer.json

@@ -18,7 +18,7 @@
   "require-dev": {
     "doctrine/coding-standard": "^13.0",
     "phpstan/phpstan": "^2.1.17",
-    "phpunit/phpunit": "^12.1.5"
+    "phpunit/phpunit": "^12.1.6"
   },
   "autoload": {
     "psr-4": {