Rename master branch to main

[ivanvc]

For consistency with the rest of the etcd repositories, and because of DEI, I think it would be good to rename the master branch to main.

[ahrtr]

I understood the intent.

But based on my previous experience of renaming bbolt master branch to main, some contributors, even including some members, are still using master in their local dev environment years after the renaming.

So I won't bother to do it. But I am not object it if other maintainers want to do it.