Bump slevomat/coding-standard from 8.3.0 to 8.4.0 in /tools/phpcs

Bumps [slevomat/coding-standard](https://github.com/slevomat/coding-standard) from 8.3.0 to 8.4.0.
- [Release notes](https://github.com/slevomat/coding-standard/releases)
- [Commits](slevomat/coding-standard@8.3.0...8.4.0)

---
updated-dependencies:
- dependency-name: slevomat/coding-standard
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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