Rename the main branch

[dingo-d]

Currently, the main branch of this repo is named master and all the other projects are using trunk or main as the main branches instead of master. We should do the same.

The docs are using some form of automatic deployment to the handbook, and I reckon that that automatic way pulls the changes from the master branch, so that part should be updated accordingly (otherwise the update process would be broken).

I tried checking the handbook meta plugin, but I didn't find anything useful so anybody with information on how this is done can comment here. I don't mind creating a meta patch for this.

[jrfnl]

Also mentioned/discussed in #66

Info on the change for the handbook update workflow can be found here: https://github.com/WordPress/wpcs-docs/pull/66#issuecomment-1114686803