Use 'trunk' instead of 'master' branch

[nielslange]

Describe the bug

Automattic is standardizing on trunk for the primary(?) branch in repos, to avoid the unfortunate connotations of the term master (see twitter threads for more background: mislav; Una Kravets).

The plan is to align Woo and other A8C products to use the name trunk (core, extension etc.). This will also align with the direction the WordPress project is taking.

Logging an issue for changing this in this repo for visibility of when and how the change happens.

We'll need to update branch protection.

We'll likely use a process similar to:

• Add a new trunk branch on a local clone.
• Push trunk to GitHub repo.
• Update default branch settings (if needed)
• Update branch protection settings (if any)

At this point, the change is made. Anyone working on the repo should now use trunk.

Update any docs to reflect the change as well.

See also:

• https://dev.to/horus_kol/renaming-your-master-branch-2a37
• https://www.hanselman.com/blog/EasilyRenameYourGitDefaultBranchFromMasterToMain.aspx

[nielslange]

@jconroy: I already pushed the trunk branch to this repo. Due to missing permissions, I wasn't able to define it as default branch and delete the master branch then. That said, I updated all PRs, and they're now pointing to trunk.

[nielslange]

The master branch had successfully been replaced with the trunk branch.