Release skill improvements

Notes from the 18.10 release activity for improvements to the release skill:

> - Combine the merge-flow and branding update PR (main) together so that there's only a single GH PR
> - .NET Eng source channel was wrong (wanted to use Arcade 10) -> should use whatever main has
> - Fix skill wording: `delete-subscriptionS` not singular
> - Phase 3 is gated on PR merging -> NOT IDEAL: It can already start to at least open the PR.
> - Finding opt-prof data is currently using a hardcoded version string in the skill / script. Update for the new format.
> - Update the skill to not submit GH PRs as draft.
> - We don't need 4.1 anymore. There are no PublicAPI.Unshipping.txt / Shipping files.