Migrate to Gatsby v4

[donovanglover]

Now that Gatsby 4 is out of beta, migrating this starter from v3 to v4 would be nice. Looks like some changes are necessary to fix the errors after updating.

More information: https://www.gatsbyjs.com/docs/reference/release-notes/migrating-from-v3-to-v4/

[jimmybutton]

Thanks for mentioning! I'll have a look at this when I have a bit more time.