Fix BaseBranchOperator import on Airflow 2 without standard provider

[vincbeck]

The standard provider is only an optional extra of the providers that use these classes, so a deployment on Airflow 2 can legitimately not have it installed. Falling back to the Airflow 2 core module keeps the compat layer resolvable with just the apache-airflow dependency it already declares, matching how every other standard-provider entry in this layer behaves.

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