alias U2 -> U2_(band)

BUR #14053 has been approved by @nonamethanks.

rename u2 -> u2_(band)

partly so someone in the future doesn't think this tag is some weird typo or such, and is an intended tag

Probably a better reason would be to disambiguate it from the Lockheed U-2 spy plane the band is named after (should we ever get an illustration of one here).

This should also be a rename, an alias doesn't solve much.

The bulk update request #14053 (forum #229038) has been approved by @nonamethanks.