Deprecate passing execution_date to XCom methods

[ashb]

As part of AIP-39 (released in 2.2) we added run_id parameters to XCom
methods, and this changes it so that passing by run_id is the
recommended approach.

A future PR will change the columns on the xcom table to store run_id
(instead/as well as exeuction_date) but that will be for 2.3, where as
this change can be backported to 2.2.x

Discussion thread: https://lists.apache.org/thread/gofj3g6m6vvksy6n0cmgq1qxd309bbbl

[uranusjr]

I’ve made all tests work and migrated most existing XCom usages to use run_id. One (very big) exception is operator link; since get_link is publicly subclassable, we can’t just change the argument. The current plan (after discussing with @ashb) is to add an optional flag on the class to toggle the behaviour, but there are much detail (and likely bikeshedding) around that, so I’m going to do it in a separate PR. The depracation warning implemented in this PR on passing execution_date to XCom is changed from DeprecationWarning to PendingDeprecationWarning to reflect that—we won’t commit the deprecation until we provide a migration path for operator links (which should still happen in 2.3, but let’s not jump the gun).

I also want to extract typing changes I made here into a separate PR, and merge that first before I mark this PR as ready for review.

[uranusjr]

Merging in the Mypy and Sphinx fixes, let’s see this works.

[uranusjr]

Oh everything is 🟢 now!

[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.

[kaxil]

Should we add a note in UPDATING.md too plz around the deprecation

[uranusjr]

UPGRADING.md entry added. I think we also want to migrate XCom to add the run_id column in 2.3, and the upgrading note will probably be changed when that is implemented.