build(deps-dev): bump stylelint from 14.16.1 to 16.20.0 (#3040)

* build(deps-dev): bump stylelint from 14.16.1 to 16.20.0

Bumps [stylelint](https://github.com/stylelint/stylelint) from 14.16.1 to 16.20.0.
- [Release notes](https://github.com/stylelint/stylelint/releases)
- [Changelog](https://github.com/stylelint/stylelint/blob/main/CHANGELOG.md)
- [Commits](stylelint/stylelint@14.16.1...16.20.0)

---
updated-dependencies:
- dependency-name: stylelint
  dependency-version: 16.20.0
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

* refactor: removed unnecessary package

* refactor: added new rule

* refactor: added configuration to ignore node_modules folder

* refactor: optimized (S)CSS code by stylelint aspects

* refactor(test): auto-format codebase

* Update README.md

* Update README.md

* Update index.css

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Maximilian Franzke <Maximilian.Franzke@deutschebahn.com>
Co-authored-by: Maximilian Franzke <787658+mfranzke@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: 3 people

.stylelintignore

@@ -0,0 +1 @@
+node_modules/

.stylelintrc.json

@@ -1,12 +1,10 @@
 {
-  "extends": [
-    "stylelint-config-standard-scss",
-    "stylelint-config-prettier-scss"
-  ],
+  "extends": ["stylelint-config-standard-scss"],
   "rules": {
     "at-rule-no-unknown": null,
     "scss/at-rule-no-unknown": true,
     "scss/at-extend-no-missing-placeholder": null,
-    "scss/at-mixin-argumentless-call-parentheses": null
+    "scss/at-mixin-argumentless-call-parentheses": null,
+    "declaration-property-value-no-unknown": true
   }
 }