Rename `master` to `main'

[dwelsch-esi]

This is following most other CNCF repos. For details see:

• Renaming the default branch from master
• The Inclusive Naming Initiative
• Cloud Native Computing Foundation’s Inclusive Naming Initiative

We've done so recently for in-toto.

[Denish3436]

@namkyu1999 @dwelsch-esi could you please assign it to me