Rename convergence branch: master -> main

[cpu]

I think this can be done in a minimally invasive fashion & I've had good experience doing this with other GitHub projects. Extant clones should work as they do today, with git printing a message about the new branch name.

[cpu]

cpu assigned FiloSottile and unassigned cpu

@FiloSottile I think this one requires elevated access. Would you mind doing it, or temporarily giving me higher perms? 🙇

[cpu]

This has been done.