fix: upgrade @mdx-js/mdx from 1.6.10 to 1.6.13

Snyk has created this PR to upgrade @mdx-js/mdx from 1.6.10 to 1.6.13.

See this package in npm:
https://www.npmjs.com/package/@mdx-js/mdx

See this project in Snyk:
https://app.snyk.io/org/github-newrelic/project/868cac7a-942a-4cb5-a87b-05381c8ca7ec?utm_source=github&utm_medium=upgrade-pr

package.json

@@ -5,7 +5,7 @@
   "dependencies": {
     "@emotion/core": "^10.0.28",
     "@emotion/styled": "^10.0.27",
-    "@mdx-js/mdx": "^1.6.10",
+    "@mdx-js/mdx": "^1.6.13",
     "@mdx-js/react": "^1.6.10",
     "@newrelic/gatsby-theme-newrelic": "^1.6.0",
     "classnames": "^2.2.6",