Modernize ConnectionFactoryBase.cs

[rklec]

• use nullable assertion, you may probably want to set <Nullable>enable</Nullable> in your project https://learn.microsoft.com/en-us/dotnet/csharp/nullable-migration-strategies
• merged many things into a pattern, see https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/patterns (suggested by Resharper)
• use new switch expression https://learn.microsoft.com/en-US/dotnet/csharp/language-reference/operators/switch-expression

Note the constructor of ConnectionFactoryBase is still quite heavy and does a lot of things. I've left it for now, but maybe needs adjustments. Also you may want to use https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-10.0/file-scoped-namespaces for the whole project.