Fix async signature (#334)

### Motivation and Context

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Incorrect controller route/method signature causing build error

### Description

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These notes will help understanding how your code works. Thanks! -->
```
D:\a\chat-copilot\chat-copilot\webapi\Controllers\MaintenanceController.cs(50,55): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread. [D:\a\chat-copilot\chat-copilot\webapi\CopilotChatWebApi.csproj]
```
### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [Contribution
Guidelines](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

webapi/Controllers/MaintenanceController.cs

@@ -1,7 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System.Threading;
-using System.Threading.Tasks;
 using CopilotChat.WebApi.Auth;
 using CopilotChat.WebApi.Hubs;
 using CopilotChat.WebApi.Models.Response;
@@ -47,7 +46,7 @@ public class MaintenanceController : ControllerBase
     [HttpGet]
     [ProducesResponseType(StatusCodes.Status200OK)]
     [ProducesResponseType(StatusCodes.Status400BadRequest)]
-    public async Task<ActionResult<MigrationResult?>> GetMaintenanceStatusAsync(
+    public ActionResult<MigrationResult?> GetMaintenanceStatus(
         [FromServices] IKernel kernel,
         [FromServices] IHubContext<MessageRelayHub> messageRelayHubContext,
         CancellationToken cancellationToken = default)