[AIRFLOW-6860] Default ignore_first_depends_on_past to True (#7610)

UPDATING.md

@@ -86,7 +86,7 @@ with DAG('my_dag):
 
 ### Deprecating ignore_first_depends_on_past on backfill command and default it to True
 
-When doing backfill with `depends_on_past` dags, users will need to pass `ignore_first_depends_on_past`.
+When doing backfill with `depends_on_past` dags, users will need to pass `--ignore-first-depends-on-past`.
 We should default it as `true` to avoid confusion
 
 ### Custom executors is loaded using full import path

airflow/cli/commands/dag_command.py

@@ -68,7 +68,7 @@ def dag_backfill(args, dag=None):
     signal.signal(signal.SIGTERM, sigint_handler)
 
     import warnings
-    warnings.warn('--ignore_first_depends_on_past is deprecated as the value is always set to True',
+    warnings.warn('--ignore-first-depends-on-past is deprecated as the value is always set to True',
                   category=PendingDeprecationWarning)
 
     if args.ignore_first_depends_on_past is False: