Bump phpmd/phpmd from 2.14.1 to 2.15.0 in /tools/phpmd

Bumps [phpmd/phpmd](https://github.com/phpmd/phpmd) from 2.14.1 to 2.15.0.
- [Release notes](https://github.com/phpmd/phpmd/releases)
- [Changelog](https://github.com/phpmd/phpmd/blob/master/CHANGELOG)
- [Commits](phpmd/phpmd@2.14.1...2.15.0)

---
updated-dependencies:
- dependency-name: phpmd/phpmd
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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