Update gatsby dependency to v3

[Pyrax]

Thanks for the pull request!

It still needs some fixes to work with the specified theme-ui version. Also, as this requires a new major version for the theme, I am going to make a few more changes to benefit from new features. I'll keep track of the progress here:

• [x] update to Emotion 11 required by theme-ui remove Emotion dependency as it is not used
• [x] theme-ui: adjust to new color modes no adjustment needed
• [x] theme-ui: rename Styled to Themed
• [x] update React to ^17.x.x to use jsxImportSource instead of old JSX Pragma (edit: seems to need additional configuration which I don't want to require from theme users)
• [x] update react-netlify-forms
• [x] check if @mdx-js/react is still needed and remove if not

[Pyrax]

New version 3.0.0 now uses Gatsby v3.

(Checks failed because of current monorepo setup not resolving to the local package version but instead uses the NPM published one - nothing to do with the changes here)