build(deps-dev): bump gatsby-plugin-postcss from 4.12.0 to 4.13.0 (#200)

Bumps [gatsby-plugin-postcss](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-postcss) from 4.12.0 to 4.13.0.
- [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/[email protected]/packages/gatsby-plugin-postcss)

---
updated-dependencies:
- dependency-name: gatsby-plugin-postcss
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

package.json

@@ -78,7 +78,7 @@
     "gatsby-plugin-minify-html": "^1.0.4",
     "gatsby-plugin-nprogress": "^3.13.0",
     "gatsby-plugin-offline": "^4.13.0",
-    "gatsby-plugin-postcss": "^4.11.0",
+    "gatsby-plugin-postcss": "^4.13.0",
     "gatsby-plugin-react-helmet": "^4.13.0",
     "gatsby-plugin-robots-txt": "^1.6.9",
     "gatsby-plugin-sitemap": "^4.9.0",