Bump gatsby-plugin-react-helmet from 3.1.18 to 3.1.21

Bumps [gatsby-plugin-react-helmet](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-react-helmet) from 3.1.18 to 3.1.21.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-react-helmet/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/[email protected]/packages/gatsby-plugin-react-helmet)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

yarn.lock

@@ -10716,9 +10716,9 @@ gatsby-plugin-page-creator@^2.1.36:
     micromatch "^3.1.10"
 
 gatsby-plugin-react-helmet@^3.0.12, gatsby-plugin-react-helmet@^3.1.3:
-  version "3.1.18"
-  resolved "https://registry.yarnpkg.com/gatsby-plugin-react-helmet/-/gatsby-plugin-react-helmet-3.1.18.tgz#093b5d92e50dafa837ae97ac2672d79a96feb201"
-  integrity sha512-wwKH6Z5NK0Gl2hDfMnWNGE41GjWpZGq0eKczPydTJc/1SbPykBFCBrjYgrAPB2ZrRnA8AV06cRwPddJC0mY7sw==
+  version "3.1.21"
+  resolved "https://registry.yarnpkg.com/gatsby-plugin-react-helmet/-/gatsby-plugin-react-helmet-3.1.21.tgz#659f31020295f42d017e2eb03175d73516ab824d"
+  integrity sha512-6LZ2LEYTwqD+ZqyCH55mVpk2xEXbQoCTfijP1W4ZCQsKtpWGJP+vyd6b96FWVyEb2k5LsQ1u+jk4R8xXULSX+w==
   dependencies:
     "@babel/runtime" "^7.7.6"