Make run_after non public and move validation logic for the payloas to the Serializer

Make run_after non-public. 

I think we need to make run_after non public. Feels like we should not expose this to the user as an option -- but rather, it's something that the scheduler should figure out based on the dag / dag run.

_Originally posted by @dstandish in https://github.com/apache/airflow/issues/46616#issuecomment-2651110552_
            