chore: setup v5 release channel

[wardpeet]

Description

Setup Gatsby v5 alpha channel. It's similar to our v4 release. We merge everything in master behind _CFLAGS_.MAJOR, this allows us to make breaking changes when publishing v5 but keeping v4 channel working.

We should use _CFLAGS_.MAJOR for all things that's javascript based. For other related files we can use git patches that auto apply in order.

[LekoArts]

This here also needs an update, right?

gatsby/packages/babel-preset-gatsby-package/lib/index.js

Line 14 in 7fe8e51

COMPILER_OPTIONS = `GATSBY_MAJOR=4`,

[LekoArts]

@TylerBarnes What about https://github.com/gatsbyjs/gatsby/blob/7fe8e511b0dcd1707eefca2c63b0f27174d7252c/packages/gatsby-source-wordpress/src/utils/gatsby-version.ts, can this be changed to just behave normal now?

[wardpeet]

The above we want to keep as is; it's the default that we run with.