GH-81746
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Validate extension JSON: API was removed: classes/EditorSceneFormatImporterFBX

Renamed to EditorSceneFormatImporterFBX2GLTF.

The compat breakage was deemed necessary as this is a class most users wouldn't
use directly, and the name needs to be disambiguated with the new
EditorSceneFormatImporterUFBX.