Use initdb instead of migratedb to handle database initialization/upgrade

[FrancisLee13]

Overview

Currently, the database initialization/upgrade is done by calling airflow's migratedb in db_command.py. migratedb internally calls airflow's upgradedb, which has more parameters than initdb does, including some parameters with default values. Using migratedb requires us to provide all of parameters, and we also need to make sure the parameters we provide are consistent across airflow versions, whereas using initdb we don't need to provide any parameters, and we can depend on the default(which is, in most cases, recommended) behaviour of airflow when doing db migration/initialization.

Acceptance Criteria

In the next version release, change the migratedb call to initdb in file <version>/python/database/migrate.py

Additional Info

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[FrancisLee13]

initdb in airflow/cli/commands/db_command.py is deprecated, but the same function in airflow/utils/db.py is not marked as deprecated.

When we work on the next Airflow version release, we should confirm what should be the recommended way.