NuGet updates the TargetFrameworks according to the documentation's support requirements.

[minbaby]

原因：文档上写支持.NET Framework 4.6.1+ or .NET Core 3.1+ / .NET 5.0+, 但是 NuGet 支持的 Framework 是 net8。

工作：

1. TargetFramework: 从 net8.0 变更为 net8.0;net5.0;netcoreapp3.1;net462
2. 更新 readme 支持的 .NET Framework 从 4.6.1+ 升级为 4.6.2+， 因为 System.IO.Ports 9.0.3 最低支持 4.6.2
3. 增加 LangVersion，修复 Error CS8400 : 功能“全局 using 指令”在 C# 8.0 中不可用。请使用语言版本 10.0 或更高版本。

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Reason: The document states that it supports ".NET Framework 4.6.1+ or .NET Core 3.1+ / .NET 5.0+". However, NuGet supports the Framework of "net8".
Work:

1. TargetFramework: Changed from net8.0 to net8.0;net5.0;netcoreapp3.1;net462
2. The supported .NET Framework in the readme has been upgraded from 4.6.1+ to 4.6.2+, as System.IO.Ports 9.0.3 has a minimum requirement of 4.6.2.
3. The LangVersion has been added and the error "Error CS8400: The feature 'Global Using Directive' is not available in C# 8.0. Please use a language version 10.0 or higher." has been fixed. `