Always show backfill button but enable it only on dags with schedule.

[tirkarthi]

Closes #50904

[pierrejeambrun]

Looking good, thanks

[github-actions]

Backport successfully created: v3-0-test

Status	Branch	Result
✅	v3-0-test

[jscheffl]

Mhm, I just see this change and... actually I dis-like it. I undrstand that people seeking for (the new cool feature to backfill as advertised) and can not see it if no schedule.

BUT: The new UI uses a lot more whitespace and when using the trigger form as modal a lot of screen space is occupied for a selection that is actually not a benefit if you just want to trigger.
In our environment 80% of DAGs are without schedule. For our users it might rather be confusing having this option when they never thought of backfill.

Could we make this somehow configurable? In my view it just clutters the UI when no backfill possible.

[pierrejeambrun]

Later on the UI will also handle the permissions, which means that this button will most likely disappear again if users do not have permissions to create backfill. Maybe that could be a workaround => Don't give user backfill permissions if you don't want them to see the disabled button. (But that's 3.1.0+ unfortunately).