Update Avalonia.ReactiveUI to ReactiveUI.Avalonia

[Mateusz-Nejman]

Due to [AvaloniaUI/Avalonia/issues/19601](Splat issue) there is a need to replace Avalonia.ReactiveUI with ReactiveUI.Avalonia in newer version of Avalonia.

[OmidID]

Hello @Mateusz-Nejman
Thanks for contributing.

I try your PR and I got this error below:

System.InvalidOperationException: Call from invalid thread
   at Avalonia.Threading.Dispatcher.<VerifyAccess>g__ThrowVerifyAccess|16_0()
   at Avalonia.Threading.Dispatcher.VerifyAccess()
   at Avalonia.AvaloniaObject.VerifyAccess()
   at Avalonia.AvaloniaObject.GetValue[T](StyledProperty`1 property)
   at Avalonia.Controls.Button.get_Command()
   at Avalonia.Controls.Button.CanExecuteChanged(Object sender, EventArgs e)
   at ReactiveUI.ReactiveCommandBase`2.OnCanExecuteChanged(Boolean newValue) in c:\temp\releaser\ReactiveUI\src\ReactiveUI\ReactiveCommand\ReactiveCommandBase.cs:line 144
   at System.Reactive.AnonymousSafeObserver`1.OnNext(T value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.IdentitySink`1.OnNext(T value)
   at System.Reactive.Subjects.FastImmediateObserver`1.EnsureActive(Int32 count)
   at System.Reactive.Subjects.FastImmediateObserver`1.EnsureActive()
   at System.Reactive.Subjects.ReplaySubject`1.ReplayBase.OnNext(T value)
   at System.Reactive.Subjects.ReplaySubject`1.OnNext(T value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.IdentitySink`1.OnNext(T value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.Linq.ObservableImpl.DistinctUntilChanged`2._.OnNext(TSource value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.Linq.ObservableImpl.CombineLatest`3._.SecondObserver.OnNext(TSecond value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.IdentitySink`1.OnNext(T value)
   at System.Reactive.Subjects.FastImmediateObserver`1.EnsureActive(Int32 count)
   at System.Reactive.Subjects.FastImmediateObserver`1.EnsureActive()
   at System.Reactive.Subjects.ReplaySubject`1.ReplayBase.OnNext(T value)
   at System.Reactive.Subjects.ReplaySubject`1.OnNext(T value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.IdentitySink`1.OnNext(T value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.Linq.ObservableImpl.DistinctUntilChanged`2._.OnNext(TSource value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.IdentitySink`1.OnNext(T value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.Linq.ObservableImpl.Select`2.Selector._.OnNext(TSource value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.Linq.ObservableImpl.Scan`2._.OnNext(TSource value)
   at System.Reactive.SafeObserver`1.WrappingSafeObserver.OnNext(TSource value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.ObserveOnObserverLongRunning`1.Drain()
   at System.Reactive.ObserveOnObserverLongRunning`1.<>c.<.cctor>b__17_0(ObserveOnObserverLongRunning`1 self, ICancelable cancelable)
   at System.Reactive.Concurrency.DefaultScheduler.LongRunning.LongScheduledWorkItem`1.<>c.<.ctor>b__3_0(Object thisObject)
   at System.Reactive.Concurrency.ConcurrencyAbstractionLayerImpl.<>c.<StartThread>b__8_0(Object itemObject)
   at System.Threading.Thread.StartCallback()

This is happen after I click LOGIN! button on startup page.