Update master branch references to main where suitable

[consideRatio]

This is the first step to renaming from master to main, approve and I'll get the rest done.

[yuvipanda]

thanks, @consideRatio!

I'm very much in favor :)

The one question I have is - does this mean that downstream users of this action would need to rename master to main in their actions? or is the compatibility / rename layer take care of that?

[consideRatio]

The one question I have is - does this mean that downstream users of this action would need to rename master to main in their actions? or is the compatibility / rename layer take care of that?

I expect this to work for anyone referencing master still, but I'm not 100% confident. I suggest we test and see, and think about how to handle it otherwise.

[consideRatio]

Blah... hmmm...

Error: Unable to resolve action `jupyterhub/repo2docker-action@master`, unable to find version `master`

[consideRatio]

I think the options, like providing a mirrored branch, or providing a moving tag master always pointing to main's HEAD are too messy - reverting this.