Remove unused TriggerRule.DUMMY

[shahar1]

TriggerRule.DUMMY has no effect in Airflow 3, so it could be safely removed.

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Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
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In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[shahar1]

Unfortunately, TriggerRule.DUMMY is still operable in AIrflow 2, so I can't backport this one to v2-10-test. As I plan to make some compatible changes in the trigger rule depedencies - I want to avoid code deviations for now between the branches in this area.
For that reason, I'm drafting this PR for now.

[eladkal]

Also needs news fragment as this is a breaking change

[shahar1]

Unfortunately, TriggerRule.DUMMY is still operable in AIrflow 2, so I can't backport this one to v2-10-test. As I plan to make some compatible changes in the trigger rule depedencies - I want to avoid code deviations for now between the branches in this area. For that reason, I'm drafting this PR for now.

In a second thought, the compatible changes shouldn't affect the trigger_rule.py file (no expected changes in the other statuses), so it should be ok to merge.
I added a newsfragment (I had to obscure the content it due to check-for-inclusive-language 😅)