### Description

Currently, the `airflow variables export` command requires an explicit file path and does not support output to stdout. However connections can be printed to stdout using `airflow connections export -`. This inconsistency between the two export commands can lead to confusion and limits the flexibility of the variables export command.

### Use case/motivation

To bring some consistency, similar to connections, variables should also be printed to STDOUT, using `-` instead of filename. 

### Related issues

_No response_

### Are you willing to submit a PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

"Migrate the schema of the metadata database. "
            "Create the database if it does not exist "
            "To print but not execute commands, use option ``--show-sql-only``. "
            "If using options ``--from-revision`` or ``--from-version``, you must also use "
            "``--show-sql-only``, because if actually *running* migrations, we should only "
            "migrate from the *current* Alembic revision."
        ),
        func=lazy_load_command("airflow.cli.commands.db_command.migratedb"),
        args=(
            ARG_DB_REVISION__UPGRADE,
            ARG_DB_VERSION__UPGRADE,
            ARG_DB_SQL_ONLY,
            ARG_DB_FROM_REVISION,
            ARG_DB_FROM_VERSION,
            ARG_DB_RESERIALIZE_DAGS,
            ARG_VERBOSE,
        ),
    ),
    ActionCommand(
        name="downgrade",
        help="Downgrade the schema of the metadata database.",
        description=(
            "Downgrade the schema of the metadata database. "
            "You must provide either `--to-revision` or `--to-version`. "
            "To print but not execute commands, use option `--show-sql-only`. "
            "If using options `--from-revision` or `--from-version`, you must also use `--show-sql-only`, "
            "because if actually *running* migrations, we should only migrate from the *current* Alembic "
            "revision."
        ),
        func=lazy_load_command("airflow.cli.commands.db_command.downgrade"),