Fix edge cases of "migrate/create-default-connections"

[potiuk]

In #32810, "airflow db migrate" command has been added and it
is used by start-airflow command. There were a few edge cases
not covered and this PR completes it.

• We can move the "database/load_default_connections" configuration
  to a new "deprecated" section. This is the first time we remove
  the option completely as it lost its meaning, but we likely
  still want to explain that the option was there and what it does
  when deprecated "db init" command is used.

It has no meaning when you use "airflow db migrate" or when you run the
new "airflow connections create-default-connections" commands. So we can
now remove it completely from configuraiton. It will still work
in the "airflow db init" which is deprecated, as long as we provide an
explicit fallback. Also if someone had it defined in their config or
env variable, it will continue to work even if it is not defined.

• We need to explain the change in a significant newsfragment.

• The start-airflow command supports creating default connections
  with --load-default-connections flag. This was lost after
  the change so this PR brings it back by running the new
  "airflow connections create-default-connections" command if the
  flag is used.

• The start-airflow breeze command can be used to start older
  versions of airflow - with ``--use-airflow-version" - those that do not
  support airflow db migrate command. In this case the old behaviour is
  used with setting the "AIRFLOW__DATABASE__LOAD_DEFAULT_CONNECTIONS"
  based on the flag passed and running "airflow db init" instead.

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
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In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[potiuk]

cc: @Adaverse -> small follow up after #32810 - noticed a few edge-cases and I think we need to add release notes to explain the new behaviour.

[potiuk]

OK. Changed the approach. Seems to be nicer and less surprising. Options can now be individually deprecated (not replaced by others but just deprecated) - and deprecation reason is displayed nicely in both - warning message and in the documentation.

[jedcunningham]

The formatting is a little weird to me. Having the first line of the reason in the blue box feels unnecessary.

[potiuk]

The formatting is a little weird to me. Having the first line of the reason in the blue box feels unnecessary.

Yeah. Sphinx is doing things on its own sometimes. I had another verison of it that looked different. Let me try it.

[potiuk]

How about this?

(other things except description -> detail done. I might want to change it in a separate PR though.

[potiuk]

Same flaky as fixed (?) in main.