Multicopter velocity control gains backwards compatibility depends on hover thrust

[MaEtUgR]

Describe the bug
The rescaling of the velocity controller with the backwards compatibility term depending on the hover thrust combined with the hover thrust estimator adjusting the hover thrust: https://github.com/PX4/Firmware/blob/master/src/modules/mc_pos_control/PositionControl/PositionControl.cpp#L151

I still think it's at least part of what we've seen in #14640

The goal was the exact opposite to get less dependency and more reproducible tuning but trying to remain backwards compatible it depends now even more on the hover thrust.

Expected behavior
The scale should be adjusted by a constant hover thrust independent factor for compatibility and then the parameter replaced with a rescaled version applying the constant factor for automatic conversion.

Additional context
#14212