fix: upgrade multiple dependencies with Snyk

Snyk has created this PR to upgrade:
  - gatsby-plugin-manifest from 2.4.18 to 2.4.19.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-manifest
  - gatsby-plugin-sharp from 2.6.19 to 2.6.20.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-sharp
  - gatsby-source-filesystem from 2.3.19 to 2.3.20.
    See this package in npm: https://www.npmjs.com/package/gatsby-source-filesystem

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

@@ -15,19 +15,19 @@
     "gatsby-image": "^2.4.13",
     "gatsby-plugin-emotion": "^4.3.10",
     "gatsby-plugin-layout": "^1.3.10",
-    "gatsby-plugin-manifest": "^2.4.18",
+    "gatsby-plugin-manifest": "^2.4.19",
     "gatsby-plugin-mdx": "^1.2.26",
     "gatsby-plugin-meta-redirect": "^1.1.1",
     "gatsby-plugin-offline": "^3.2.17",
     "gatsby-plugin-react-helmet": "^3.3.10",
     "gatsby-plugin-robots-txt": "^1.5.1",
     "gatsby-plugin-sass": "^2.3.12",
-    "gatsby-plugin-sharp": "^2.6.19",
+    "gatsby-plugin-sharp": "^2.6.20",
     "gatsby-plugin-sitemap": "^2.4.11",
     "gatsby-plugin-use-dark-mode": "^1.1.2",
     "gatsby-remark-autolink-headers": "^2.3.11",
     "gatsby-remark-images": "^3.3.17",
-    "gatsby-source-filesystem": "^2.3.18",
+    "gatsby-source-filesystem": "^2.3.20",
     "gatsby-transformer-remark": "^2.8.23",
     "gatsby-transformer-sharp": "^2.5.10",
     "js-cookie": "^2.2.1",