I think 12.1 version supports only following features for STP SNMP trap
snmp-server enable traps :
To enable SNMP notifications (traps or informs), use the snmp-server enable traps command. To disable all SNMP notifications, use the no form of this command.
snmp-server enable traps [flash [insertion | removal] | fru-ctrl | port-security [trap-rate trap-rate] | removal | stpx | vlancreate | vlandelete | vtp] [mac-notification [change | move | threshold]
no snmp-server enable traps [flash [insertion | removal] | fru-ctrl | port-security [trap-rate trap-rate] | removal | stpx | vlancreate | vlandelete | vtp] [mac-notification]
if you want more features you should upgrade the higher versions.