THRIFT-5407: Minor changes in .NET to support netcoreapp3.1

[emmenlau]

This PR re-adds support for .NET CORE 3.1. It still requires latest .NET CORE to build. Users need to manually switch the build from net5.0 to netcoreapp3.1. This can be done in DotNet or directly in the csproj files.

• Apache Jira ticket https://issues.apache.org/jira/browse/THRIFT-5407
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