fix: upgrade gatsby-plugin-mdx from 1.3.1 to 1.4.0

Snyk has created this PR to upgrade gatsby-plugin-mdx from 1.3.1 to 1.4.0.

See this package in npm:


See this project in Snyk:
https://app.snyk.io/org/github-newrelic/project/ee280047-f651-440a-aaec-462aac2b4fe2?utm_source=github&utm_medium=upgrade-pr

package.json

@@ -18,7 +18,7 @@
     "gatsby-plugin-emotion": "^4.3.14",
     "gatsby-plugin-layout": "^1.3.12",
     "gatsby-plugin-manifest": "^2.4.35",
-    "gatsby-plugin-mdx": "^1.2.47",
+    "gatsby-plugin-mdx": "^1.4.0",
     "gatsby-plugin-meta-redirect": "^1.1.1",
     "gatsby-plugin-newrelic": "newrelic/gatsby-plugin-newrelic.git#5cd42e832f0588764dc332ce7ebd05794de5106a",
     "gatsby-plugin-offline": "^3.2.32",