fix: upgrade @mdx-js/react from 1.6.17 to 1.6.18

Snyk has created this PR to upgrade @mdx-js/react from 1.6.17 to 1.6.18.

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

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

@@ -6,7 +6,7 @@
     "@emotion/core": "^10.0.35",
     "@emotion/styled": "^10.0.27",
     "@mdx-js/mdx": "^1.6.17",
-    "@mdx-js/react": "^1.6.17",
+    "@mdx-js/react": "^1.6.18",
     "@newrelic/gatsby-theme-newrelic": "^1.9.5",
     "@splitsoftware/splitio-react": "^1.2.0",
     "@xstate/react": "^1.0.0-rc.6",