Bump clean-css from 4.1.11 to 4.2.1

Bumps [clean-css](https://github.com/jakubpawlowicz/clean-css) from 4.1.11 to 4.2.1.
- [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@v4.1.11...v4.2.1)

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

Author: dependabot-support

yarn.lock

@@ -1314,11 +1314,11 @@ class-utils@^0.3.5:
     static-extend "^0.1.1"
 
 clean-css@^4.1.9:
-  version "4.1.11"
-  resolved "https://registry.yarnpkg.com/clean-css/-/clean-css-4.1.11.tgz#2ecdf145aba38f54740f26cefd0ff3e03e125d6a"
-  integrity sha1-Ls3xRaujj1R0DybO/Q/z4D4SXWo=
+  version "4.2.1"
+  resolved "https://registry.yarnpkg.com/clean-css/-/clean-css-4.2.1.tgz#2d411ef76b8569b6d0c84068dabe85b0aa5e5c17"
+  integrity sha512-4ZxI6dy4lrY6FHzfiy1aEOXgu4LIsW2MhwG0VBKdcoGoH/XLFgaHSdLTGr4O8Be6A8r3MOphEiI8Gc1n0ecf3g==
   dependencies:
-    source-map "0.5.x"
+    source-map "~0.6.0"
 
 cli-cursor@^2.1.0:
   version "2.1.0"
@@ -4602,12 +4602,12 @@ source-map-url@^0.4.0:
   resolved "https://registry.yarnpkg.com/source-map-url/-/source-map-url-0.4.0.tgz#3e935d7ddd73631b97659956d55128e87b5084a3"
   integrity sha1-PpNdfd1zYxuXZZlW1VEo6HtQhKM=
 
-source-map@0.5.x, source-map@^0.5.3, source-map@^0.5.6, source-map@^0.5.7:
+source-map@^0.5.3, source-map@^0.5.6, source-map@^0.5.7:
   version "0.5.7"
   resolved "https://registry.yarnpkg.com/source-map/-/source-map-0.5.7.tgz#8a039d2d1021d22d1ea14c80d8ea468ba2ef3fcc"
   integrity sha1-igOdLRAh0i0eoUyA2OpGi6LvP8w=
 
-source-map@^0.6.0, source-map@^0.6.1, source-map@~0.6.1:
+source-map@^0.6.0, source-map@^0.6.1, source-map@~0.6.0, source-map@~0.6.1:
   version "0.6.1"
   resolved "https://registry.yarnpkg.com/source-map/-/source-map-0.6.1.tgz#74722af32e9614e9c287a8d0bbde48b5e2f1a263"
   integrity sha512-UjgapumWlbMhkBgzT7Ykc5YXUT46F0iKu8SGXq0bcwP5dz/h0Plj6enJqjz1Zbq2l5WaqYnrVbwWOWMyF3F47g==