[C#12] reduce namespace indentation

[emako]

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/namespace

use namespace FluentModbus; to reduce indentation.

[emako]

@Apollo3zehn

[Apollo3zehn]

Please create a P/R, I would love to use the simplified namespace definition in this project.

[emako]

All right, create a PR later