Update documentation and code to default to 'main' branch instead of 'master'

[codezninja]

This would be a breaking change.

To support with standards coming across tech industry we should change defaults from master to main instead.

Currently we have support with people to start using it ConditionalApplyPlugin.withApplyOnBranch('myMasterReplacement')

[kmanning]

I actually was not aware of this change. I dug around a bit, and found this: https://github.com/github/renaming <-- I'm assuming this is what you're referring to?

1. I've been pushing large breaking changes to a v6.0 release. v6.0 seems like a good candidate for changing the default - part of the v6.0 can include a migration guide, which can detail the changes that one would need to make to adjust to the breaking changes.
2. In the meantime, depending on how fast we want to move, I could imagine the ConditionalApplyPlugin supporting both "master"/"main" simultaneously in a v5.* release. <-- that wouldn't exactly be a breaking change, and therefore I'd be slightly more inclined to release that sooner as a minor release, especially if we can think of a clever way of disabling it if it caused some sort of unexpected problem. (eg: what if someone had a project that had both master and main, and they wanted them to be treated differently for some reason. This feels like a corner case that I'd be ok with causing a "break" that could be fixed by some minor change).

[codezninja]

I think we can simplify it and got with 1. and release it as part of v6.0 as a breaking default change.

What I think we should do, in the next release have a deprecation message saying master will be replace with main in v6.0.

To get ahead of the curve, you can implement ConditionalApplyPlugin.withApplyOnBranch('main') so people can fix any issues ahead of the release cause I'm sure they have to make other changes related to it as well. That way when v6.0 comes out, you can just remove ConditionalApplyPlugin.withApplyOnBranch('main') and it would work out of the box