Drop execution_date unique constraint on DagRun

[uranusjr]

The column has also been renamed to logical_date, although the Python model is not changed. This allows us to not need to fix all the Python code at once (we'll do that later), but still do the two changes in one migration instead of two.

Note that this one is not breaking—the metadatabase interface is not user-facing, we’re only removing a check that stopped impossible things.

[sunank200]

I think the tests has to change as well

[uranusjr]

Fixed. If this passes, I think it can be merged first.

[kaxil]

It isn’t breaking change but might be worth mentioning in Towncrier notes as a note-worthy change if someone is using combination of dag_id & execution_date to retrieve DagRuns

[uranusjr]

I think it is more appropriate to attach the news fragment on the next PR that will rename the attribute on DagRun instead. That one is the actual breaking change most people will notice.

This PR only affects the SQL layer, not Python. Having separate fragments for changes in each layer will likely be a bit confusing.