adds build required OSPlatform calls

[ebozduman]

This PR fixes the offending RuntimeInformation.IsOSPlatform(OSPlatform.Windows) calls as shown in the build logs below:

Error: /home/runner/work/minio-dotnet/minio-dotnet/FileUploader/FileUpload.cs(35,16): error MA0144: Use System.OperatingSystem to check the current OS (https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0144.md) [/home/runner/work/minio-dotnet/minio-dotnet/FileUploader/FileUploader.csproj]
Error: /home/runner/work/minio-dotnet/minio-dotnet/Minio.Functional.Tests/FunctionalTest.cs(4833,29): error MA0144: Use System.OperatingSystem to check the current OS (https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0144.md) [/home/runner/work/minio-dotnet/minio-dotnet/Minio.Functional.Tests/Minio.Functional.Tests.csproj::TargetFramework=net6.0]
Error: /home/runner/work/minio-dotnet/minio-dotnet/Minio.Examples/Program.cs(285,13): error MA0144: Use System.OperatingSystem to check the current OS (https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0144.md) [/home/runner/work/minio-dotnet/minio-dotnet/Minio.Examples/Minio.Examples.csproj]
  Minio.Tests -> /home/runner/work/minio-dotnet/minio-dotnet/Minio.Tests/bin/Release/net6.0/Minio.Tests.dll
  Successfully created package '/home/runner/work/minio-dotnet/minio-dotnet/artifacts/Minio.6.0.2.nupkg'.
  Successfully created package '/home/runner/work/minio-dotnet/minio-dotnet/artifacts/Minio.6.0.2.snupkg'.
Error: /home/runner/work/minio-dotnet/minio-dotnet/Minio.Functional.Tests/FunctionalTest.cs(4833,29): error MA0144: Use System.OperatingSystem to check the current OS (https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0144.md) [/home/runner/work/minio-dotnet/minio-dotnet/Minio.Functional.Tests/Minio.Functional.Tests.csproj::TargetFramework=net7.0]