Target Calling Branch for Contributing Update

[ColmBhandal]

The contributing-update action should target whatever branch it was called on in the ensuing PRs. This supports both uses cases:

1. If it's called on main, e.g. as part of a cron job, and there have been updates to the template since the last call, then the ensuing PR will target main as it should.
2. If it's called on a branch that's updating contributing_inputs.yaml, then it will target that branch, meaning the ensuing PR only need be merged into that branch - avoiding cumbersome manual steps such as merging two PRs or manually applying changes to both the contributing_inputs.yaml and the contributing.md files.

The README for the action must be updated to reflect this change.