Consider usage of .NET 9 Blazor APIs for authentication state management

[josephdecock]

.NET 9 added some new types related to authentication, and we might want to use those too.

See https://learn.microsoft.com/en-us/aspnet/core/release-notes/aspnetcore-9.0?view=aspnetcore-9.0#simplified-authentication-state-serialization-for-blazor-web-apps

Probably we don't want to do anything here, because we want to support both .NET 8 and 9 for now, but we should take a look.

[Erwinvandervalk]

@josephdecock, I'm moving this to milestone 'bff-future' for exactly the reason you describe (the blazor .net 8 compatibility). These blazor api improvements are only in .net 9 and would do the same thing as we're doing here. It's more code to maintain, but keeps it compatible.

We can reconsider once we drop support for blazor 8.