Bump nth-check and gatsby-plugin-mdx

Bumps [nth-check](https://github.com/fb55/nth-check) to 2.1.1 and updates ancestor dependency [gatsby-plugin-mdx](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-mdx). These dependencies need to be updated together.


Updates `nth-check` from 2.0.0 to 2.1.1
- [Release notes](https://github.com/fb55/nth-check/releases)
- [Commits](fb55/nth-check@v2.0.0...v2.1.1)

Updates `gatsby-plugin-mdx` from 2.14.2 to 4.3.0
- [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@4.3.0/packages/gatsby-plugin-mdx)

---
updated-dependencies:
- dependency-name: nth-check
  dependency-type: indirect
- dependency-name: gatsby-plugin-mdx
  dependency-type: direct:production
...

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