Rename "master" branch to "main" for glibc 2.35 release.

[Florian Weimer]

* Carlos O'Donell:

> My proposal is to rename the development and release branches at the point
> that glibc 2.35 branches:
>
> * master -> main
>
> * release/2.35/master -> release/2.35/main
>
> No alias would be provided for the master branch; we would immediately
> start using 'main' as the development branch.

What is the reason for using ”main” for release branches?

If we don't add a symbolic ref, we could use release/2.35.

Thanks,
Florian