Bump clean-css from 4.1.9 to 4.1.11 in /client_src

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>

client_src/yarn.lock

@@ -1370,8 +1370,8 @@ clap@^1.0.9:
     chalk "^1.1.3"
 
 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"