Update trunk config

When setting up to a new computer, trunk complains it can't find a "default" branch named main or master.

Prior setups probably not affected if one of those branch previously existed.

Error: Unable to detect an upstream commit on this branch: Unable to detect a trunk branch such as 'main'. Checked the following - refs/remotes//HEAD, refs/remotes//main, refs/remotes//master, main, master

Reference: https://docs.trunk.io/check/reference/trunk-yaml#repo