Bump grunt-stylelint from 0.20.1 to 0.21.0

Bumps [grunt-stylelint](https://github.com/wikimedia/grunt-stylelint) from 0.20.1 to 0.21.0.
- [Release notes](https://github.com/wikimedia/grunt-stylelint/releases)
- [Changelog](https://github.com/wikimedia/grunt-stylelint/blob/main/History.md)
- [Commits](https://github.com/wikimedia/grunt-stylelint/commits)

---
updated-dependencies:
- dependency-name: grunt-stylelint
  dependency-version: 0.21.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

package.json

@@ -22,7 +22,7 @@
     "grunt-contrib-watch": "1.1.0",
     "grunt-replace-regex": "1.0.3",
     "grunt-sass": "4.1.0",
-    "grunt-stylelint": "0.20.1",
+    "grunt-stylelint": "0.21.0",
     "grunt-terser": "2.0.0",
     "load-grunt-tasks": "5.1.0",
     "postcss": "8.5.9",