Change deprecation warning to debug logging
Change deprecation warning to debug logging

This prevents a confusing warning from being logged to the console (and
to the log file upon each salt-minion daemon restart).

It seems that the intent of the deprecation warning was to use the logic
in `warn_until` to force us to make the final deprecation when the
Magnesium release cycle arrives, but when we do these sort of
deprecations we are typically pro-active and do a grep on the release
codename. So, keeping "Magnesium" in the log message should be
sufficient to remind us of this.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt