Set catchup_by_default config to False by default

[TJaniF]

"Oh I need to unpause the DAG to run it, gotcha"
click
Many DAG runs spawn and start running
"Nonono I only wanted one, stop stop"

⬆️ This was me almost 3 years ago, using Airflow for the first time, obviously not understanding scheduling yet, and thinking of catchup mainly as a condiment.

Now I know how start_date works, but I still often set it just a while in the past because time math can be hard. Setting catchup=False has become a default and I don't think I've written more than one or two DAGs without it, ever.

Talking to other Airflow users, my experience seems to be the common one, especially when first starting with Airflow, accidental catchup as a rite of passage.

Preparing this PR I also think the vast majority of example DAGs have catchup=False, it's the de-facto default, so lets make it the default? :)

Side note: The tests appear to use explicit setting of catchup when testing it, so no adjustment needed there as far as I can tell.

EDIT: got breeze to run and it isnt working yet... 👀
EDIT2: got it to work but I touched dags.py 😅

DIscussion: https://lists.apache.org/thread/omh35gs8r5rmcxhs6onygzfhh9znkodm
Lazy Consensus: https://lists.apache.org/thread/mdpsyrodctv2ky2pvsl605y85h5mo8hv

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[eladkal]

If we do this we need newsfrgment.

I would also perfer to have discussion/lazy consensus in the mailing list. Previously we had some related suggestion to the catchup issue in general
#35392 (comment)
#38168

[uranusjr]

Looks like a lot of tests are not happy because they rely on the first run being created against start_date. Let’s just add catchup_by_default = true in airflow/config_templates/unit_tests.cfg so tests still set catchup by default.

[uranusjr]

Not enough knowledge on the lint command and the config default, otherwise lgtm.

[uranusjr]

Also need to fix static checks. Kubernetes test failures are unrelated I believe.

[TJaniF]

Not enough knowledge on the lint command and the config default, otherwise lgtm.

Thank you @uranusjr! The static check was an extra empty line, sorry, fixed that now :)
And yes, I don't think the K8s checks are failing because of something I did... 🤔

EDIT: it is 💚 ! :D