Add pre-upgrade check for dangling TI references

[dstandish]

We are adding some foreign keys in 2.3.0 so we want make it more likely that migration succeeds by detecting FK violations and moving the records out of the table before creating the FK. We already had a check for dag run but this adds one for TaskInstance.

[dstandish]

i have not tested on all 4 but it's very similar to the dagrun check so it's probably fine. i will confirm tomorrow.

tested on all 4 dbs

Test data:

testing from pre-2.2.0: test_2_1_4.txt
testing upgrade from 2.2.0 < ver < 2.3.0: test_2_2_5.txt

procedure:

delete and recreate db
airflow db upgrade --to-version 2.1.4
insert test data
airflow db upgrade

[dstandish]

OK @ashb @uranusjr i think this is ready for a look.

Recall that prior to this PR, we had "missing dag run" checks for dangling rows. But this wasn't quite all that we needed. It made sense for TaskInstance, when we added run_id to it in 2.2.0. However, for the other tables we are changing in 2.3.0, checking for the existence of a DagRun is not actually strict enough, since we're keying to TI -- we need to look for a record in TI.join(DR).

Initially I thought I should add a second set of checks -- for TI -- alongside the DR checks. But, since the check for dangling TI ref implies a check for dangling DR ref, we can skip the DR check and just check for TI. This enables us to do all checks (missing DR and missing TI) in the same loop, though this required a bit of a refactor.

One complication is that when db < 2.2.0 the join conditions for TI.join(DR) are different from when 2.2.0 <= db < 2.3.0. Initially I was going to read airflow version of db for this determination, but it ended up being easy enough to use a "duck typing" inference approach.

[uranusjr]

Seems good to me

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.