rename default branch to main

[marcoieni]

Related to rust-lang/infra-team#227

Migration guide:

• confirm that no other rust-lang projects hardcode the master branch name (stdarch maintainer)
• approve this PR (stdarch maintainer)
• approve team PR stdarch: rename default branch to main team#2068 (another infra admin)
• Change the branch name in the GitHub settings of the repo (Marco)
• merge this PR (Marco)
• merge team PR stdarch: rename default branch to main team#2068 (Marco)

[rustbot]

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[folkertdev]

confirm that no other rust-lang projects hardcode the master branch name

The only repo that we interact with is rust-lang/rust, but you know more about the infra there than we do probably.

[sayantn]

Only place I can see something going wrong is the josh sync thing with r-l/rust repo - although I don't really know too much to comment

[folkertdev]

We discussed this a bit here #t-infra > Renaming default branch of r-l/stdarch from master to main, I think the ball is back in T-infra's court?