### Body

Currently `start_date` is mandatory parameter.
For DAGs with `schedule=None` we can relax this requirement as no scheduling calculation needed so the `start_date` parameter isn't used.

### Committer

- [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.
warnings.warn(
            "DAG.full_filepath is deprecated in favour of fileloc",
            RemovedInAirflow3Warning,
            stacklevel=2,
        )
        self.fileloc = value
    @property
    def concurrency(self) -> int:
        
        warnings.warn(
            "The 'DAG.concurrency' attribute is deprecated. Please use 'DAG.max_active_tasks'.",
            RemovedInAirflow3Warning,
            stacklevel=2,
        )
        return self._max_active_tasks
    @concurrency.setter
    def concurrency(self, value: int):
        self._max_active_tasks = value
    @property
    def max_active_tasks(self) -> int:
        return self._max_active_tasks
    @max_active_tasks.setter
    def max_active_tasks(self, value: int):