tools(deps-dev): Update icanhazstring/composer-unused requirement (#374)

Updates the requirements on [icanhazstring/composer-unused](https://github.com/composer-unused/composer-unused) to permit the latest version.
- [Release notes](https://github.com/composer-unused/composer-unused/releases)
- [Changelog](https://github.com/composer-unused/composer-unused/blob/main/CHANGELOG.md)
- [Commits](composer-unused/composer-unused@0.8.8...0.8.9)

---
updated-dependencies:
- dependency-name: icanhazstring/composer-unused
  dependency-type: direct:development
...

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

Author: dependabot[bot]

tools/composer-unused/composer.json

@@ -3,7 +3,7 @@
     "type": "metapackage",
     "description": "composer-unused",
     "require-dev": {
-        "icanhazstring/composer-unused": "0.8.8",
+        "icanhazstring/composer-unused": "0.8.9",
         "roave/security-advisories": "dev-latest"
     },
     "config": {