AP_ExternalAHRS: stamp last_location_update_us in MicroStrain7 driver

Every other ExternalAHRS backend records last_location_update_us when
it writes state.location; the MicroStrain7 driver never did.  That
left get_location's velocity-based position extrapolation - intended
to cope with slow backends - permanently disabled for MicroStrain7,
so the reported position was stale by up to one filter-packet
interval.

Mirror the MicroStrain5 driver.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

libraries/AP_ExternalAHRS/AP_ExternalAHRS_MicroStrain7.cpp

@@ -200,6 +200,7 @@ void AP_ExternalAHRS_MicroStrain7::post_filter() const
         // Use GNSS 0 even though it may be bad.
         state.location = Location{filter_data.lat, filter_data.lon, gnss_data[0].msl_altitude, Location::AltFrame::ABSOLUTE};
         state.have_location = true;
+        state.last_location_update_us = AP_HAL::micros();
 
         state.quat = filter_data.attitude_quat;
         state.have_quaternion = true;