AUTO_LAND: Require fully valid local position again

[mbjd]

Solved Problem

The current mode requirements allow FlightTaskAuto to start with invalid local position. But regardless of failsafe_flags.local_position_invalid_relaxed, FlightTask populates its _position with NaN when it is invalid in the proper sense. This then leads to various problems such as:

• erroneously disallowed nudging due to the MPC_LAND_RADIUS check returning NaN
• not even publishing a trajectory setpoint from FlightModeManager::generateTrajectorySetpoint due to another position validity check that does not know the concept of local_position_invalid_relaxed

Solution

This reverts the mode requirements to valid local position again. This causes the existing FlightTaskDescend to be engaged when position goes invalid, so that nudging inputs are still executed, even at the cost of losing any position stabilisation.

This reverts commit 2f69f3f, PR #23917.

Longer term, the functionality aimed at in #23917 should probably be made available with a separate FlightTask that works with invalid position, but valid velocity estimates.

[hamishwillee]

If this changes, may need update to https://docs.px4.io/main/en/flight_modes_mc/land.html (usually the top info section)

[mbjd]

• Mode requires at least a valid local position estimate (does not require a global position).
  • Flying vehicles can't switch to this mode without valid local position.
  • Flying vehicles will failsafe if they lose the position estimate.

The docs make no mention of the mode supporting invalid but not invalid_relaxed local position, also there was no docs update accompanying the original #23917. Therefore I think the docs are actually more correct after the proposed change than before it :)