build(deps): bump postcss from 7.0.21 to 7.0.23 (#470)

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.21 to 7.0.23.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/7.0.21...7.0.23)

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

yarn.lock

@@ -5883,9 +5883,9 @@ postcss-selector-parser@^6.0.2:
     uniq "^1.0.1"
 
 postcss@^7.0.21:
-  version "7.0.21"
-  resolved "https://registry.yarnpkg.com/postcss/-/postcss-7.0.21.tgz#06bb07824c19c2021c5d056d5b10c35b989f7e17"
-  integrity sha512-uIFtJElxJo29QC753JzhidoAhvp/e/Exezkdhfmt8AymWT6/5B7W1WmponYWkHk2eg6sONyTch0A3nkMPun3SQ==
+  version "7.0.23"
+  resolved "https://registry.yarnpkg.com/postcss/-/postcss-7.0.23.tgz#9f9759fad661b15964f3cfc3140f66f1e05eadc1"
+  integrity sha512-hOlMf3ouRIFXD+j2VJecwssTwbvsPGJVMzupptg+85WA+i7MwyrydmQAgY3R+m0Bc0exunhbJmijy8u8+vufuQ==
   dependencies:
     chalk "^2.4.2"
     source-map "^0.6.1"