Bump gatsby-plugin-postcss from 2.3.6 to 2.3.11

Bumps [gatsby-plugin-postcss](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-postcss) from 2.3.6 to 2.3.11.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-postcss/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-postcss@2.3.11/packages/gatsby-plugin-postcss)

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

Author: dependabot-preview[bot]

yarn.lock

@@ -5243,9 +5243,9 @@ gatsby-plugin-css-modules-flow-types@^0.1.1:
     css-modules-flow-types-loader "^1.1.1"
 
 gatsby-plugin-postcss@^2.0.2:
-  version "2.3.6"
-  resolved "https://registry.yarnpkg.com/gatsby-plugin-postcss/-/gatsby-plugin-postcss-2.3.6.tgz#d0d8b19f9edf845b54ea49a2cdd9ceb2a52f10f5"
-  integrity sha512-rJ7GihxGc6pA3KapR2kFPYkDyVc/18Yt2nROxL893epv2nYn99H2XaeLOFvglr6FrpFkJP0TvUvwDOfQOuTz8A==
+  version "2.3.11"
+  resolved "https://registry.yarnpkg.com/gatsby-plugin-postcss/-/gatsby-plugin-postcss-2.3.11.tgz#e02e3e00f37192263b3c5eb290b1d856771e57d4"
+  integrity sha512-IHZ2s0iLspL3spYKpFz4SUqQHPPImvPhvVAFEBkGNRA65ffmgOLqucsaXzINjFcpdy5o3+dS+GJuMePaSGYIbg==
   dependencies:
     "@babel/runtime" "^7.10.3"
     postcss-loader "^3.0.0"