Purge existing SLA implementation

[ferruzzi]

As discussed in the Airflow 3.0 dev calls, the first step for the new SLA system is stripping the existing implementation out.

I did this PR in multiple commits to try to make it more transparent. In this PR:

1. Remove everything related to SLA in the code.
2. Add back the absolute minimum so that existing DAGs won't throw an exception. @bbovenzi said he can add a UI banner so any DAG which implements an SLA will get a notification that SLA is not working in 3.0, but we don't want all existing DAGs to start throwing exceptions left and right either.
3. Added a log message to models/dag which triggers if the DAG is init'ed with an "sla_miss_callback"
4. Went through the docs directory and stripped out all references to the SLA feature.

[o-nikolas]

There are is an SLA related configuration that I don't see removed in this PR?
https://airflow.apache.org/docs/apache-airflow/stable/configurations-ref.html#check-slas

[jscheffl]

Ah, added the breaking label, now CI test fails with exactly the comment I wanted to make - please add a newsfragment for the breaking change
Otherwise - looks good for me.

I am a bit surprised that the carve-out of SLA just gains 1k LoC

[ferruzzi]

I am a bit surprised that the carve-out of SLA just gains 1k LoC

Yeah. It's an odd one. I really expected this to be a much larger surgery with tendrils all over the place. It wasn't as bad as I had feared; 28 files is pretty broad but a solid chunk of them are docs pages which just linked back tot he SLA concept page.

please add a newsfragment for the breaking change

I should have known to do that, sorry. I'll add it.

There are is an SLA related configuration that I don't see removed in this PR?

Yeah, I may have missed that one. I'll make another pass and see what I can find related to "check_slas" and prune it.

Maybe a sentence or two describing this feature as deprecated in 3.0? And that there are plans to add it back in 3.1?

I'll come up with something.