Move LOAD_DEFAULT_CONNECTIONS env var to database config section in CI

[josh-fell]

As of 2.3 the load_default_connections config was moved from "core" to a new "database" section. Currently when spinning up Airflow in Breeze a DeprecationWarning is displayed for this config:

Starting Airflow

DB: postgresql+psycopg2://postgres:***@postgres/airflow
[2022-06-18 03:05:09,528] {db.py:1471} INFO - Creating tables
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
/opt/airflow/airflow/configuration.py:526 DeprecationWarning: The load_default_connections option in [core] has been moved to the load_default_connections option in [database] - the old setting has been used, but please update your config.
Initialization done

Likely not all of the changes in this PR directly affect the DeprecationWarning thrown when using the breeze start-airflow command, but for the sake of due diligence, the remaining environment variables of AIRFLOW__CORE__LOAD_DEFAULT_CONNECTIONS have been updated to AIRFLOW__DATABASE__LOAD_DEFAULT_CONNECTIONS.

---

^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragement file, named {pr_number}.significant.rst, in newsfragments.