chore(deps): update mdx monorepo to v2

Author: renovate-bot

package.json

@@ -8,8 +8,8 @@
     "url": "https://github.com/gatsbyjs/gatsby/issues"
   },
   "dependencies": {
-    "@mdx-js/mdx": "1.6.22",
-    "@mdx-js/react": "1.6.22",
+    "@mdx-js/mdx": "2.0.0",
+    "@mdx-js/react": "2.0.0",
     "babel-plugin-styled-components": "2.0.5",
     "gatsby": "3.14.6",
     "gatsby-image": "3.11.0",