Rename AU921 -> AU915

[terrillmoore]

Per @paulalting #340:

We all here in Australia call our band AU915, not AU921.

Rename CFG_au921 to CFG_au915, and do the required in all the subordinate locations. Allow CFG_au921 to imply CFG_au915 with a warning.

[Scobber]

I concur. it appears to be named incorrectly.

AU_923 is not a thing.
The formal band names in use here in Australia are AU_915 and AS_921, and the latter is supposed to support the equivalent of 923 as a sub band.