chore: bump stylelint from 16.20.0 to 16.21.0

Bumps [stylelint](https://github.com/stylelint/stylelint) from 16.20.0 to 16.21.0.
- [Release notes](https://github.com/stylelint/stylelint/releases)
- [Changelog](https://github.com/stylelint/stylelint/blob/main/CHANGELOG.md)
- [Commits](stylelint/stylelint@16.20.0...16.21.0)

---
updated-dependencies:
- dependency-name: stylelint
  dependency-version: 16.21.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

package-lock.json

@@ -40,7 +40,7 @@
     "eslint": "^9.29.0",
     "sinon": "^21.0.0",
     "sinon-chrome": "^3.0.1",
-    "stylelint": "^16.20.0",
+    "stylelint": "^16.21.0",
     "stylelint-config-standard": "^38.0.0",
     "stylelint-no-unsupported-browser-features": "^8.0.4",
     "web-ext": "^8.7.1"