ResourceManagerWithCultureStringLocalizer class and WithCulture interface member to be removed

[ryanbrandenburg]

Following up on #346.

The ResourceManagerWithCultureStringLocalizer class and WithCulture interface member were often sources of confusion for users of Localization, especially if they wanted to create their own IStringLocalizer implementation. These items give the user the impression that we expect an IStringLocalizer instance to be "per-language, per-resource", when actually they should only be "per-resource", with the language searched for determined by the CultureInfo.CurrentUICulture at execution time. To remove this source of confusion and to avoid APIs which we don't want users to use these were obsoleted in 3.0.0-preview3, and they are removed in 5.0.0.

For context, see #346 and dotnet/aspnetcore#3324.

See dotnet/aspnetcore#7756 for discussion.

[PriyaPurkayastha]

@scottaddie for awareness since this will need to be documented for .NET 5
@ryanbrandenburg can you please remove the "Documented" label - I don't have access to remove it? Scott adds that label after he publishes this to .NET Core docs.