Bump gatsby-plugin-mdx from 1.0.74 to 2.14.1

Bumps [gatsby-plugin-mdx](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-mdx) from 1.0.74 to 2.14.1.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/gatsby-plugin-mdx@2.14.1/packages/gatsby-plugin-mdx/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-mdx@2.14.1/packages/gatsby-plugin-mdx)

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

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

packages/gatsby-theme/package.json

@@ -29,7 +29,7 @@
     "gatsby-plugin-catch-links": "^2.1.0",
     "gatsby-plugin-compile-es6-packages": "^2.0.0",
     "gatsby-plugin-emotion": "^4.1.0",
-    "gatsby-plugin-mdx": "^1.0.13",
+    "gatsby-plugin-mdx": "^2.14.1",
     "gatsby-plugin-react-helmet": "^3.1.0",
     "gatsby-plugin-theme-ui": "^0.3.0",
     "gatsby-remark-import-code": "^0.1.1",