chore(deps): bump gatsby-plugin-postcss from 2.3.12 to 2.3.13 in /client

Bumps [gatsby-plugin-postcss](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-postcss) from 2.3.12 to 2.3.13.
- [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@2.3.13/packages/gatsby-plugin-postcss)

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

client/package.json

@@ -38,7 +38,7 @@
     "gatsby-plugin-advanced-sitemap": "^1.5.6",
     "gatsby-plugin-create-client-paths": "^2.3.13",
     "gatsby-plugin-manifest": "^2.4.34",
-    "gatsby-plugin-postcss": "^2.3.12",
+    "gatsby-plugin-postcss": "^2.3.13",
     "gatsby-plugin-react-helmet": "^3.3.14",
     "gatsby-plugin-remove-serviceworker": "^1.0.0",
     "gatsby-remark-prismjs": "^3.5.16",