Gatsby: [v2] Upgrade three official starters on their v2 branch

[KyleAMathews]

I probably won't get to this today so if someone else wants to work on it, that'd be great!

• [x] upgrade three official starters on their v2 branch
• [x] link to them from the announcement blog post (#6073)
• [x] link to them from the upgrade guide on next.gatsbyjs.org
• [x] link to them from the quickstart on next.gatsbyjs.org (#6070)
• [x] link to them from the readme (#6070)
• [x] link to them in the “Life After Layouts” post (#5816)

Edit: created checklist from issue title, shortened title

[mottox2]

I updated gatsby-starter-default to v2.
Please create v2 branch to gatsby-starter-default repository to make pull request.

https://github.com/gatsbyjs/gatsby-starter-default/compare/master...mottox2:v2?expand=1

[jlengstorf]

@mottox2 I just created a v2 branch: https://github.com/gatsbyjs/gatsby-starter-default/tree/v2

Thank you!

[m-allanson]

Note - gatsby new can parse branch names, so you can install v2 versions of starters. Check out the v2 tutorial for an example of this.

[jlengstorf]

A link to that tutorial: https://next.gatsbyjs.org/tutorial/part-one/#install-the-hello-world-starter

[KyleAMathews]

This work is done. Thanks for everyone's help!