Bump phpunit/phpunit in the development-dependencies group

Bumps the development-dependencies group with 1 update: [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit).


Updates `phpunit/phpunit` from 9.6.33 to 12.5.8
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/12.5.8/ChangeLog-12.5.md)
- [Commits](sebastianbergmann/phpunit@9.6.33...12.5.8)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-version: 12.5.8
  dependency-type: direct:development
  update-type: version-update:semver-major
  dependency-group: development-dependencies
...

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

Author: dependabot[bot]

composer.json

@@ -31,6 +31,6 @@
 	},
 	"require-dev": {
 		"wp-coding-standards/wpcs": "^3.0",
-		"phpunit/phpunit": "^9"
+		"phpunit/phpunit": "^13"
 	}
 }