Update hosted-services.md to reflect BackgroundService.StartAsync breaking change (#37054)

* Update hosted-services.md to reflect BackgroundService.StartAsync breaking change

As of dotnet/runtime#116283 (in .NET 10), `BackgroundService.StartAsync` no longer blocks until the `BackgroundService.ExecuteAsync` implementation reaches an `await` point; `StartAsync` now always runs `ExecuteAsync` on the thread pool using `Task.Run`.

* Fix updated doc to only apply to >= .NET 10

Author: dionrhys

aspnetcore/fundamentals/host/hosted-services.md

@@ -80,8 +80,22 @@ The hosted service is activated once at app startup and gracefully shut down at
 
 <xref:Microsoft.Extensions.Hosting.BackgroundService> is a base class for implementing a long running <xref:Microsoft.Extensions.Hosting.IHostedService>.
 
+:::moniker-end
+
+:::moniker range=">= aspnetcore-10.0"
+
+[ExecuteAsync(CancellationToken)](xref:Microsoft.Extensions.Hosting.BackgroundService.ExecuteAsync%2A) is called on the thread pool to run the background service. The implementation returns a <xref:System.Threading.Tasks.Task> that represents the entire lifetime of the background service. The host blocks in [StopAsync(CancellationToken)](xref:Microsoft.Extensions.Hosting.BackgroundService.StopAsync%2A) waiting for `ExecuteAsync` to complete.
+
+:::moniker-end
+
+:::moniker range=">= aspnetcore-8.0 < aspnetcore-10.0"
+
 [ExecuteAsync(CancellationToken)](xref:Microsoft.Extensions.Hosting.BackgroundService.ExecuteAsync%2A) is called to run the background service. The implementation returns a <xref:System.Threading.Tasks.Task> that represents the entire lifetime of the background service. No further services are started until [ExecuteAsync becomes asynchronous](https://github.com/dotnet/extensions/issues/2149), such as by calling `await`. Avoid performing long, blocking initialization work in `ExecuteAsync`. The host blocks in [StopAsync(CancellationToken)](xref:Microsoft.Extensions.Hosting.BackgroundService.StopAsync%2A) waiting for `ExecuteAsync` to complete.
 
+:::moniker-end
+
+:::moniker range=">= aspnetcore-8.0"
+
 The cancellation token is triggered when [IHostedService.StopAsync](xref:Microsoft.Extensions.Hosting.IHostedService.StopAsync%2A) is called. Your implementation of `ExecuteAsync` should finish promptly when the cancellation token is fired in order to gracefully shut down the service. Otherwise, the service ungracefully shuts down at the shutdown timeout. For more information, see the [IHostedService interface](#ihostedservice-interface) section.
 
 For more information, see the [BackgroundService](https://github.com/dotnet/runtime/blob/main/src/libraries/Microsoft.Extensions.Hosting.Abstractions/src/BackgroundService.cs) source code.