fix: upgrade multiple dependencies with Snyk

Snyk has created this PR to upgrade:
  - gatsby from 5.0.0 to 5.12.4.
    See this package in npm: https://www.npmjs.com/package/gatsby
  - gatsby-plugin-feed from 5.0.0 to 5.12.0.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-feed
  - gatsby-plugin-manifest from 5.0.0 to 5.12.0.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-manifest
  - gatsby-plugin-sharp from 5.0.0 to 5.12.0.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-sharp
  - gatsby-remark-images from 7.0.0 to 7.12.0.
    See this package in npm: https://www.npmjs.com/package/gatsby-remark-images
  - gatsby-remark-prismjs from 7.0.0 to 7.12.0.
    See this package in npm: https://www.npmjs.com/package/gatsby-remark-prismjs
  - gatsby-remark-responsive-iframe from 6.0.0 to 6.12.0.
    See this package in npm: https://www.npmjs.com/package/gatsby-remark-responsive-iframe
  - gatsby-source-filesystem from 5.0.0 to 5.12.0.
    See this package in npm: https://www.npmjs.com/package/gatsby-source-filesystem
  - gatsby-transformer-remark from 6.0.0 to 6.12.0.
    See this package in npm: https://www.npmjs.com/package/gatsby-transformer-remark
  - gatsby-transformer-sharp from 5.0.0 to 5.12.0.
    See this package in npm: https://www.npmjs.com/package/gatsby-transformer-sharp

See this project in Snyk:
https://app.snyk.io/org/sammyfilly/project/c2ba23e1-8e78-4d98-a602-4215c0203b4d?utm_source=github&utm_medium=referral&page=upgrade-pr