[Due for payment 2025-02-13] Improve cherry-pick conflict resolution experience

[roryabraham]

Problem

The existing experience to resolve conflicts when cherry-picking a PR is a bit wonky. It commits conflict markers, which should not really ever be committed in the code. It also makes it less ergonomic to resolve those conflicts, because they won't appear as such in most git/diff tools.

Solution

In cases where a CP fails due to conflicts, just commit the version bump, and include the CP command in the PR description for the engineer to run manually. Furthermore:

• Ensure the author of the original PR that's being CP'd is assigned, not just the deployer who triggered the CP
• Ensure that we include the PR template (including checklists) and enforce that checklists are completed for these kind of manual CP's

[roryabraham]

#55993 merged and announced here. Calling this done, but will reopen if there are issues

[melvin-bot]

Reviewing label has been removed, please complete the "BugZero Checklist".