Add .Net 6 support

We are using a .Net 6 app with hangfire but run into the issue that we can't use the default .net serviceprovider in .Net 6

It seems we are missing the public static IServiceCollection AddHangfireServer([NotNull] this IServiceCollection services, [NotNull] Action optionsAction) method for .Net containers in the .Net 6 as referenced here as it is hidden behind preprocessor directive that don't include the latest .Net versions: https://github.com/HangfireIO/Hangfire/blob/master/src/Hangfire.AspNetCore/HangfireServiceCollectionExtensions.cs#L107

Start in src/Hangfire.AspNetCore/HangfireServiceCollectionExtensions.cs at the AddHangfireServer overload around line 107, and inspect the preprocessor directive that hides it for .NET 6. Verify that the public IServiceCollection extension is available to a .NET 6 application and that the reported default service-provider integration can compile and use it.

Written by the indexing model from the issue text.