fix: upgrade multiple dependencies with Snyk

Snyk has created this PR to upgrade:
  - gatsby-image from 2.4.12 to 2.4.13.
    See this package in npm: https://www.npmjs.com/package/gatsby-image
  - gatsby-plugin-emotion from 4.3.9 to 4.3.10.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-emotion
  - gatsby-plugin-manifest from 2.4.17 to 2.4.18.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-manifest
  - gatsby-plugin-offline from 3.2.16 to 3.2.17.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-offline
  - gatsby-plugin-react-helmet from 3.3.9 to 3.3.10.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-react-helmet
  - gatsby-plugin-sass from 2.3.10 to 2.3.12.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-sass
  - gatsby-plugin-sharp from 2.6.18 to 2.6.19.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-sharp
  - gatsby-plugin-sitemap from 2.4.10 to 2.4.11.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-sitemap
  - gatsby-remark-images from 3.3.17 to 3.3.18.
    See this package in npm: https://www.npmjs.com/package/gatsby-remark-images
  - gatsby-source-filesystem from 2.3.18 to 2.3.19.
    See this package in npm: https://www.npmjs.com/package/gatsby-source-filesystem
  - gatsby-transformer-remark from 2.8.23 to 2.8.25.
    See this package in npm: https://www.npmjs.com/package/gatsby-transformer-remark
  - gatsby-transformer-sharp from 2.5.10 to 2.5.11.
    See this package in npm: https://www.npmjs.com/package/gatsby-transformer-sharp

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