[PM-22380] Enable NRT for some Core project files (#5912)

* Enable NRT for Core/Jobs files

* Enable NRT for Core/HostedServices files

* Enable NRT for Core/Exceptions files

* Enable NRT for Core/NotificationHub files

---------

Co-authored-by: Bernd Schoolmann <mail@quexten.com>

src/Core/NotificationHub/NotificationHubPool.cs

@@ -5,6 +5,8 @@ using Microsoft.Extensions.Logging;
 
 namespace Bit.Core.NotificationHub;
 
+#nullable enable
+
 public class NotificationHubPool : INotificationHubPool
 {
     private List<NotificationHubConnection> _connections { get; }