Add descend mode as a failsafe.

[AlexisTM]

The land mode supposes we have a GPS causing issues when flying with a local position (such as mocap or vision)
This adds the alternative to use the descend mode.

Solved Problem

When working on local position systems such as a vision localization, the land mode doesn't fit the need when the localization is failing as we want a way to land.

Furthermore, the current Land mode supposes the use of GPS which is not present using vision/flow/mocap, making the drone try to land at (0,0) with a heading of 0°, which is not the expectation.

Fixes #23773
Fixes #21524

Solution

• Add the use of Descend mode for all failsafe actions

Changelog Entry

For release notes:

Feature: Failsafe modes now include the descend mode

Alternatives

There could be an automatic demotion from Land to Descend

Context

Related links, screenshot before/after, video

[AlexisTM]

(Small messup happened in this branch)