Change type of ReturnType of IEdmOperation to use IEdmOperationReturn

[xuzhg]

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Assemblies affected

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Reproduce steps

• In the 7 version, we introduce IEdmOperationReturn, but avoid breaking change, we don't use this interface in the IEdmOperation
• and keep using IEdmTypeReference. See here: 'https://github.com/OData/odata.net/blob/main/src/Microsoft.OData.Edm/Schema/Interfaces/IEdmOperation.cs#L19'

In the next major version (9.x), we should change it to use IEdmOperationReturn.

Meanwhile, in 8.x, we should obsolete the field and add a message about that customer can use 'GetReturn()' extension method to get the 'IEdmOperationReturn' then decorate the annotation on it.

Expected result

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Actual result

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Additional detail

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