Bump flat and gatsby

Removes [flat](https://github.com/hughsk/flat). It's no longer used after updating ancestor dependency [gatsby](https://github.com/gatsbyjs/gatsby). These dependencies need to be updated together.


Removes `flat`

Updates `gatsby` from 2.23.12 to 2.32.13
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/compare/gatsby@2.23.12...gatsby@2.32.13)

---
updated-dependencies:
- dependency-name: flat
  dependency-type: indirect
- dependency-name: gatsby
  dependency-type: direct:production
...

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

package.json

@@ -26,7 +26,7 @@
     "enzyme": "^3.11.0",
     "enzyme-adapter-react-16": "^1.15.5",
     "enzyme-to-json": "^3.6.1",
-    "gatsby": "^2.23.12",
+    "gatsby": "^2.32.13",
     "gatsby-image": "^2.4.9",
     "gatsby-plugin-google-analytics": "^2.3.11",
     "gatsby-plugin-manifest": "^2.4.14",