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 2.3.5 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/[email protected]@2.32.13)

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

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

Author: dependabot[bot]