Chore: bump dependencies

[andrico1234]

My main motivation to do this was to unblock the sitemap, which requires Gatsby v3+

Updates packages to their most recent versions. I omitted some:

Todo

• regression test across the site

Notes

• Bad news, gatsby-plugin-favicon [doesn't work](favicon not good: Unexpected token u in JSON at position 0 Creatiwity/gatsby-plugin-favicon#71)
• Good news, gatsby-plugin-manifest, which was previously commented out due to a bug, works fine with Gatsby 3, and has favicon functionality
  
• sharp was removed because it wasn't used anywhere. The installation notes for gatsby-plugin-sharp don't mention needing to install sharp at all.
• theme-ui and gatsby-plugin-theme-ui were both removed as neither were used in the repo.
• husky wasn't updated because there were some major changes to the api in v6, and I wanted to keep the change surface small
• prettier wasn't updated to keep the change surface small
• the two remark plugins weren't updated to v3 as they removed the ability to import via require, gatsby doesn't allow import without a bit of faff
• i didn't update emotion because the repo is no longer available, and so I don't know what was changed in the most recent major version 🤷🏽‍♀️
• I updated the version of Node to 14. Nothing broke
• All images were broken until I made the change in Image.js. I have no idea why...

[andrico1234]

🤷🏽