Bump gatsby-plugin-mdx from 1.2.37 to 5.13.1

Bumps [gatsby-plugin-mdx](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-mdx) from 1.2.37 to 5.13.1.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-mdx/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-mdx@5.13.1/packages/gatsby-plugin-mdx)

---
updated-dependencies:
- dependency-name: gatsby-plugin-mdx
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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