[rotorcraft] guidance_v: limit z_ref in update_ref_from_zd_sp

Limit the gv_z_ref to GUIDANCE_V_REF_MAX_Z_DIFF (default 2m) from current z position.
This should be useful if the aircraft can't follow the alitude in climb mode (e.g. when limited by max throttle).
[ground_segment] server: fix kml export