[Dart] Treat float as double

Created by: sbu-WBT

Double gets special special treatment in the mustache to prevent some errors like type 'int' is not a subtype of type 'double'.

https://github.com/OpenAPITools/openapi-generator/blob/dad931a25a54d6d1500654b96499d5239240c659/modules/openapi-generator/src/main/resources/dart2/class.mustache#L75-L82

Dart has no floats so they are replaced with doubles. Before this fix, it wasn't the case inside the mustache file so the floats had the type double in the class, but not the code from the isDouble block inside the mustache because only isFloat was true.

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