composer(deps-dev): bump phpcompatibility/phpcompatibility-wp (#1740)

Bumps [phpcompatibility/phpcompatibility-wp](https://github.com/PHPCompatibility/PHPCompatibilityWP) from 2.1.5 to 2.1.6.
- [Release notes](https://github.com/PHPCompatibility/PHPCompatibilityWP/releases)
- [Commits](PHPCompatibility/PHPCompatibilityWP@2.1.5...2.1.6)

---
updated-dependencies:
- dependency-name: phpcompatibility/phpcompatibility-wp
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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