Copter: change RTL_ALT_MIN from 200cm to 30cm

[tatsuy]

In this PR, I propose lowering the minimum RTL altitude (RTL_ALT_MIN) from 2m to 30cm, making RTL operations more feasible in confined spaces, such as indoors.
To mitigate any impact on existing users who have set RTL_ALT to 0, we introduce a default safety altitude (RTL_ALT_LOW) of 2m (200cm). RTL_ALT_LOW (2m) will be used if the RTL_ALT is set to a value lower than RTL_ALT_MIN (30cm).

Changes:
The RTL_ALT_MIN is updated from 200cm to 30cm.
The range of RTL_ALT parameter is updated from 200-300000 to 30-300000 cm.
Adjusted logic in RTL operations to use RTL_ALT_LOW if RTL_ALT is set below RTL_ALT_MIN.

Reference PR:
A related discussion was previously had here, which led to an update in the description and range of the RTL_ALT parameter.

[rmackay9]

I'd prefer that we just keep it simple and reduce the minimum and then release the change as part of a major update (e.g. 4.6) when users are more likely to expect significant changes in behaviour.

So let's remove the complexity around setting to values below 30cm.

I've tagged for discussion on next Tuesday's dev call so we can get feedback from other devs so you could wait until then to see what others say.

[tatsuy]

@rmackay9 Thank you for the review.
OK, I will follow the feedback from dev call.

[muramura]

The minimum altitude should be an altitude that is not affected by ground effects.
30cm is good if it is not a problem with tiny Whoops.

In Japan, research has shown that if the length of the legs is at least three times longer than the rotor radius, they are less susceptible to ground effects.

http://k-nagatani.org/pdf/2017-Robomech-MAS-online.pdf

[rmackay9]

@muramura,

Thanks for the research paper and rule-of-thumb advice re leg length and prop size.

[rmackay9]

@tatsuy,

We discussed on the dev call and others agreed we should just keep it simple and reduce the minimum define to 30cm.

[lthall]

I am happy with what ever people want.

[tatsuy]

Thank you for the discussions and feedback.
I simply changed RTL_ALT_MIN from 200 cm to 30 cm as agreed.