fix: upgrade multiple dependencies with Snyk

Snyk has created this PR to upgrade:
  - gatsby from 5.12.4 to 5.13.0.
    See this package in npm: https://www.npmjs.com/package/gatsby
  - gatsby-plugin-manifest from 5.12.0 to 5.13.0.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-manifest
  - gatsby-plugin-sharp from 5.12.0 to 5.13.0.
    See this package in npm: https://www.npmjs.com/package/gatsby-plugin-sharp
  - gatsby-source-filesystem from 5.12.0 to 5.13.0.
    See this package in npm: https://www.npmjs.com/package/gatsby-source-filesystem
  - gatsby-transformer-sharp from 5.12.0 to 5.13.0.
    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/04a9a3ed-9fab-400c-9be0-bc625c78ecce?utm_source=github&utm_medium=referral&page=upgrade-pr