Vehicle: Add Terrain Altitude Warning

[HTRamsey]

Closes #12612

Courtesy of @jnomikos & Ascend Engineering

[jnomikos]

2.038 - critical: Fact name "altitudeWarnMaxAGL" not found in ":/json/App.SettingsGroup.json" (default:SettingsGroup::_createSettingsFact:30)

Did you forget to commit the App.SettingsGroup.json code?

[HTRamsey]

2.038 - critical: Fact name "altitudeWarnMaxAGL" not found in ":/json/App.SettingsGroup.json" (default:SettingsGroup::_createSettingsFact:30)

Did you forget to commit the App.SettingsGroup.json code?

I forgot like half the PR somehow

[DonLakeFlyer]

I looked but there isn't much here. Assume it's still missing stuff.

[HTRamsey]

Think it should be all there. It's just a warning above/below set altitudes so shouldn't be much