Change .NET SDK to use namespace base Milvus.Client

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As part of the ongoing work on the Milvus .NET SDK, we had some offline discussions on the proper nuget package name and C# namespace; the current IO.Milvus aligns with e.g. Java standards and does not align with .NET norms. There has been consensus that we'll change this to Milvus.Client.

/cc @weianweigan @stephentoub @tawalke