Allow scheduled insertion of branch builds

As per the changes in [this PR](https://github.com/dotnet/project-system/pull/8420), we currently cannot schedule insertions for branches other than `main`. This is because we need to specify the target VS branch for the insertion. Previously, we were creating an entire new release pipeline to do this, but here, we don't need to go to those lengths.

- Decide on the mechanism for mapping our branch name to VS branch name
- Could be in the pipeline .yml file or in another file or location
- Allow the Insertion stage to automatically use this mapping mechanism
- The pipeline knows the name of the branch it is building via `Build.SourceBranchName`. This value would be looked up in the mapping to determine which target VS branch to use for insertion.
- We'd need a way to prefer the pipeline parameter if provided, and only fallback to the mapping selection process when that isn't provided.
- Update the `branch-checklist.md` to include how to set up automatic branch insertion schedules and how to update the mapping mechanism