Default to Celery Task model when backend model does not exist (#14612)

closes #14586

We add this feature in #12336
but looks like for some users this attribute does not exist.

I am not sure if they are using a different Celery DB Backend or not
but even Celery > 5 contains this attribute
(https://github.com/celery/celery/blob/v5.0.5/celery/backends/database/__init__.py#L66)

and even Celery 4 but this commits use the Celery Task model when an attribute
error occurs

airflow/executors/celery_executor.py

@@ -35,7 +35,7 @@
 
 from celery import Celery, Task, states as celery_states
 from celery.backends.base import BaseKeyValueStoreBackend
-from celery.backends.database import DatabaseBackend, session_cleanup
+from celery.backends.database import DatabaseBackend, Task as TaskDb, session_cleanup
 from celery.result import AsyncResult
 from celery.signals import import_modules as celery_import_modules
 from setproctitle import setproctitle  # pylint: disable=no-name-in-module
@@ -567,7 +567,7 @@ def _get_many_from_kv_backend(self, async_tasks) -> Mapping[str, EventBufferValu
     def _get_many_from_db_backend(self, async_tasks) -> Mapping[str, EventBufferValueType]:
         task_ids = _tasks_list_to_task_ids(async_tasks)
         session = app.backend.ResultSession()
-        task_cls = app.backend.task_cls
+        task_cls = getattr(app.backend, "task_cls", TaskDb)
         with session_cleanup(session):
             tasks = session.query(task_cls).filter(task_cls.task_id.in_(task_ids)).all()