Remove deprecated get_link signature support

[uranusjr]

Operator link currently supports two signatures, one modern using TaskInstanceKey, the other old using execution_date. The old signature will have a problem after the AIP-83 amendments where None is a possibility. Airflow 3.0 is a good chance to just get rid of this problem entirely.

TODO:

• Emit deprecation warning in 2.x. Emit warning for deprecated get_link signature #46448
• News fragment
• Migration tool

[uranusjr]

Old signature:

class MyOperatorLink(BaseOperatorLink):
    def get_link(self, operator, dttm):
        ...

New signature:

class MyOperatorLink(BaseOperatorLink):
    def get_link(self, operator, *, ti_key):
        ...

Note that in the old signature, dttm is used by Airflow as a positional argument, so the argument name in the function declaration may not be dttm. In the new signature, however, the argument name must be ti_key (although it may be non-keyword-only; self, operator, ti_key should be considered new signature). So the deprecated signature detection rule should be exactly three positional arguments, the third one not named ti_key.

The migration tool should search files for subclasses of BaseOperatorLink, and check their get_link declaration.

Ideally, the tool should check all entries defined in the operator_extra_links list of a plugin, even if it does not subclass BaseOperatorLink, because Airflow does not actually require you to subclass, as long as the signature match at runtime. But this can be difficult to do, so I think it’s OK to ignore this. We always subclass BaseOperatorLink in documentation, and the non-subclass implementation works only by accident.

Relevant documentation on operator extra link: https://airflow.apache.org/docs/apache-airflow/stable/howto/define-extra-link.html

[uranusjr]

Delaying this a bit per discussion with @ashb. The task SDK work may need to change this.

[uranusjr]

Got a go-ahead. We can progress now.