Reconfigure main/v6 branches in workspaces

[nicolethoen]

Once we declare we are feature frozen on PF5, we should update the repo configurations so that main branch -> v5 and v6 branch -> main.

At this point, we can also update the npm publishing so that we are releasing 6.0.0-prerelease.x versions rather than alpha versions.

Steps on how to do this can be found in the Major release development process google doc under the To begin releasing prerelease candidates from main (retire alpha development branches) heading.

• core
• react
• org
• extensions