Document Airflow 3 requirement for Databricks deferral

[breadtitor]

Document the Airflow 3 version requirement for deferrable Databricks run-now and submit-run operators.

Airflow 3.0.x cannot fetch the Databricks connection from the Triggerer during asynchronous polling, while Airflow 3.1.0 and later handle this path in core. Documenting the boundary gives affected users an actionable upgrade path instead of suggesting an unnecessary provider-side fix.

closes: #71525

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[boring-cyborg]

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[eladkal]

Reading #71525

I am not sure if this is a doc issue. Shouldn't we check on runtime the airflow version and if it is 3.0 then alert users / fallback to the supported functionality?

cc @moomindani