We want to keep it this way so people can still make plugins work for older versions of Cura, like 3.6, where the 'api' field with just a single major version, instead of the 'supported_sdk_versions', which can be either a version-string like '7.6.0' or a list of version strings.
We've seen from experience that people do end up with broken files, due to user-modifications and whatnot. Corrupting it further won't make things better for the user.
Contributes to issue CURA-8212.
