Rename branch 'master' to 'main'

[salaxander]

Describe the solution you'd like
Following the inclusive naming initiative, I think it would be good to explore renaming the master branch to main.

It looks like GitHub has made this process quite a bit more friendly than it used to be. All open pull requests are now re-targeted (which is a huge improvement!). Docs on that are here

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Gatekeeper version:
• Kubernetes version: (use kubectl version):

[stale]

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