Feature: Move celery.default_queue to operators.default_queue to allow re-use between executors

[leonsmith]

The default_queue config option resides in the celery section.

We already have an operators config section which would be a better place to specify a default.
This also allows other executors to re-use the default queue functionality without having to set celery specific config.

closes: #14696

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[turbaszek]

Looks reasonable 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 master at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[turbaszek]

@leonsmith could you please take a look at the CI issues?

[leonsmith]

Sure @turbaszek might need a little assistance as they look pretty unrelated at a surface glance?
Is master always supposed to be green? I can re-base and see if any of them change

[leonsmith]

The test suit looked like it mostly passed last night (after a rebase onto master), 2 of the actions look like they got killed as they exited with a 137 code.
No idea why they have all reset this morning & its now saying it doesn't have permissions.

[ashb]

(non blocking change request)

[mik-laj]

@leonsmith Can you do a rebase? I would like to merge this change.

[leonsmith]

@mik-laj rebased as requested 👍