Restore support of starting mapped tasks from triggerer

### Body

#48006 disabled starting mapped tasks from triggerer because it was crashing the scheduler (https://github.com/apache/airflow/issues/47735). It was discovered late in the airlfow 3 beta process so, disabling it was a reasonable choice.

When time permits, one could look into restoring this capability, perhaps with limitations.

### Committer

- [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.

Start by reading issues #48006 and #47735 to understand why starting mapped tasks from the triggerer was disabled and how it affected the scheduler. Trace the triggerer-to-scheduler path described there and define acceptable limitations before making changes. Done means the capability is restored without reproducing the scheduler crash.

Written by the indexing model from the issue text.