Better rename `master` into `3.x` **and** rename `4.x` into `main`

[clemens-tolboom]

Something like

3.x

git checkout master
git checkout -b 3.x
git push --set-upstream origin 3.x

4.x aka new main

git checkout 4.x
git checkout -b main
git push --set-upstream origin main

[Zireael07]

Yep, planning to do this in a couple days (I seem to have caught a cold :( )

[Zireael07]

Done! You can now test the new main and yell at me/update other issues if it still doesn't work/import correctly.

[clemens-tolboom]

I'm not sure we are on the same page. You muse set new main to default if that is the 4.x and edit README accordingly. And delete the 4.x ... IIRC this can all be done on github.