Bump clean-css from 4.1.9 to 4.1.11 in /frontend

Bumps [clean-css](https://github.com/jakubpawlowicz/clean-css) from 4.1.9 to 4.1.11. - [Release notes](https://github.com/jakubpawlowicz/clean-css/releases) - [Changelog](https://github.com/jakubpawlowicz/clean-css/blob/master/History.md) - [Commits](clean-css/clean-css@v4.1.9...v4.1.11) Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

frontend/yarn.lock

@@ -1564,8 +1564,8 @@ class-utils@^0.3.5:
  static-extend "^0.1.1"
 
 clean-css@4.1.x:
- version "4.1.9"
- resolved "https://registry.yarnpkg.com/clean-css/-/clean-css-4.1.9.tgz#35cee8ae7687a49b98034f70de00c4edd3826301"
+ version "4.1.11"
+ resolved "https://registry.yarnpkg.com/clean-css/-/clean-css-4.1.11.tgz#2ecdf145aba38f54740f26cefd0ff3e03e125d6a"
  dependencies:
  source-map "0.5.x"