Add cascade to DagRun/TaskInstance relationship

[ephraimbuddy]

We currently have an issue where deleting dagruns causes a dependency error in Sqlalchemy because
the session doesn't know what to do with the related taskinstances.

This PR adds cascade so that when a dagrun is marked for deletion, the related taskinstances
are also deleted

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/flask_appbuilder/models/sqla/interface.py", line 698, in delete
    self.session.commit()
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/scoping.py", line 163, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 1046, in commit
    self.transaction.commit()
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 504, in commit
    self._prepare_impl()
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 483, in _prepare_impl
    self.session.flush()
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 2540, in flush
    self._flush(objects)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 2682, in _flush
    transaction.rollback(_capture_exception=True)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/util/langhelpers.py", line 70, in __exit__
    with_traceback=exc_tb,
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/util/compat.py", line 182, in raise_
    raise exception
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 2642, in _flush
    flush_context.execute()
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/unitofwork.py", line 422, in execute
    rec.execute(self)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/unitofwork.py", line 538, in execute
    self.dependency_processor.process_deletes(uow, states)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/dependency.py", line 547, in process_deletes
    state, child, None, True, uowcommit, False
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/dependency.py", line 604, in _synchronize
    sync.clear(dest, self.mapper, self.prop.synchronize_pairs)
  File "/usr/local/lib/python3.6/site-packages/sqlalchemy/orm/sync.py", line 88, in clear
    "column '%s' on instance '%s'" % (r, orm_util.state_str(dest))
AssertionError: Dependency rule tried to blank-out primary key column 'task_instance.dag_id' on instance '<TaskInstance at 0x7fbdca3bc3c8>

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[uranusjr]

According to documentation all implies delete so the latter is not needed. However, I don’t think we should do expunge (also implied by all). I’m also not sure about refresh-expire. Maybe a safer route would be save-update, merge, delete, delete-orphan (the first two are the default value).

https://docs.sqlalchemy.org/en/14/orm/relationship_api.html#sqlalchemy.orm.relationship.params.cascade

[ashb]

When did this start failing? Cos it has been passing for a while.

Oh new test added in #16634 that passedon PR but never worked on main :(

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[potiuk]

Oh new test added in #16634 that passedon PR but never worked on main :(

Sorry again - my fault in #17883 - missed image tagging. This has been fixed in #18433 .
Also protection for the future added in #18435