<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def upgrade(**kwargs):
    '''
    Upgrade all software. Currently not implemented

    Kwargs:
        saltenv (str): The salt environment to use. Default ``base``.
        refresh (bool): Refresh package metadata. Default ``True``.

    .. note::
        This feature is not yet implemented for Windows.

    Returns:
        dict: Empty dict, until implemented

    CLI Example:

    .. code-block:: bash

        salt '*' pkg.upgrade
    '''
    log.warning('pkg.upgrade not implemented on Windows yet')
    refresh = salt.utils.data.is_true(kwargs.get('refresh', True))
    saltenv = kwargs.get('saltenv', 'base')
    log.warning('pkg.upgrade not implemented on Windows yet refresh:%s saltenv:%s', refresh, saltenv)
    # Uncomment the below once pkg.upgrade has been implemented

    # if salt.utils.data.is_true(refresh):
    #    refresh_db()
    return {}