Deprecate IStartupTask and AddStartupTask with warnings

src/Orleans.Runtime/Hosting/SiloBuilderStartupExtensions.cs

@@ -28,6 +28,7 @@ public static class SiloBuilderStartupExtensions
         /// <returns>
         /// The provided <see cref="ISiloBuilder"/>.
         /// </returns>
+        [Obsolete("AddStartupTask is deprecated. Use BackgroundService or IHostedService instead. See https://learn.microsoft.com/dotnet/orleans/host/configuration-guide/startup-tasks for more information.", error: false)]
         public static ISiloBuilder AddStartupTask<TStartup>(
             this ISiloBuilder builder,
             int stage = ServiceLifecycleStage.Active)
@@ -51,6 +52,7 @@ public static ISiloBuilder AddStartupTask<TStartup>(
         /// <returns>
         /// The provided <see cref="ISiloBuilder"/>.
         /// </returns>
+        [Obsolete("AddStartupTask is deprecated. Use BackgroundService or IHostedService instead. See https://learn.microsoft.com/dotnet/orleans/host/configuration-guide/startup-tasks for more information.", error: false)]
         public static ISiloBuilder AddStartupTask(
             this ISiloBuilder builder,
             IStartupTask startupTask,
@@ -74,6 +76,7 @@ public static ISiloBuilder AddStartupTask(
         /// <returns>
         /// The provided <see cref="ISiloBuilder"/>.
         /// </returns>
+        [Obsolete("AddStartupTask is deprecated. Use BackgroundService or IHostedService instead. See https://learn.microsoft.com/dotnet/orleans/host/configuration-guide/startup-tasks for more information.", error: false)]
         public static ISiloBuilder AddStartupTask(
             this ISiloBuilder builder,
             Func<IServiceProvider, CancellationToken, Task> startupTask,

src/Orleans.Runtime/Lifecycle/IStartupTask.cs

@@ -1,11 +1,13 @@
-using System.Threading;
+using System;
+using System.Threading;
 using System.Threading.Tasks;
 
 namespace Orleans.Runtime
 {
     /// <summary>
     /// Defines an action to be taken after silo startup.
     /// </summary>
+    [Obsolete("IStartupTask is deprecated. Use BackgroundService or IHostedService instead. See https://learn.microsoft.com/dotnet/orleans/host/configuration-guide/startup-tasks for more information.", error: false)]
     public interface IStartupTask
     {
         /// <summary>

test/Tester/StartupTaskTests.cs

@@ -8,6 +8,8 @@
 
 using Xunit;
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace DefaultCluster.Tests
 {
     [TestCategory("BVT"), TestCategory("Lifecycle")]