[generator] Change generated code to not emit CA1305 warning.

[jpobst]

Fixes: #763

Today we generate this code:

throw new InvalidCastException (string.Format ("Unable to convert instance of type '{0}' to type '{1}'.", JNIEnv.GetClassNameFromInstance (handle), "java.code.IMyInterface"));

In newer CSC versions (.NET5+), this produces a CA1305 warning. We can avoid this warning with string interpolation:

throw new InvalidCastException ($"Unable to convert instance of type '{JNIEnv.GetClassNameFromInstance (handle)}' to type 'java.code.IMyInterface'.");