Bump braces, @primer/gatsby-theme-doctocat, gatsby and gatsby-plugin-…

…postcss

Bumps [braces](https://github.com/micromatch/braces) to 3.0.3 and updates ancestor dependencies [braces](https://github.com/micromatch/braces), @primer/gatsby-theme-doctocat, [gatsby](https://github.com/gatsbyjs/gatsby) and [gatsby-plugin-postcss](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-postcss). These dependencies need to be updated together.


Updates `braces` from 3.0.2 to 3.0.3
- [Changelog](https://github.com/micromatch/braces/blob/master/CHANGELOG.md)
- [Commits](micromatch/braces@3.0.2...3.0.3)

Updates `@primer/gatsby-theme-doctocat` from 4.7.1 to 5.1.1

Updates `gatsby` from 3.15.0 to 5.13.7
- [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]@5.13.7)

Updates `gatsby-plugin-postcss` from 4.15.0 to 6.13.1
- [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: braces
  dependency-type: indirect
- dependency-name: "@primer/gatsby-theme-doctocat"
  dependency-type: direct:production
- dependency-name: gatsby
  dependency-type: direct:production
- dependency-name: gatsby-plugin-postcss
  dependency-type: direct:production
...

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