[C#] bump: dotnet to v1.8.1 (#2180)

## Linked issues

closes: #minor

## Details
Introduce fix:
* #2178

## Attestation Checklist

- [x] My code follows the style guidelines of this project

- I have checked for/fixed spelling, linting, and other errors
- I have commented my code for clarity
- I have made corresponding changes to the documentation (updating the
doc strings in the code is sufficient)
- My changes generate no new warnings
- I have added tests that validates my changes, and provides sufficient
test coverage. I have tested with:
  - Local testing
  - E2E testing in Teams
- New and existing unit tests pass locally with my changes

### Additional information

> Feel free to add other relevant information below

---------

Co-authored-by: Lily Du <[email protected]>
Co-authored-by: lilydu <[email protected]>

dotnet/packages/Microsoft.TeamsAI/Microsoft.TeamsAI/Application/StreamingResponse.cs

@@ -313,7 +313,7 @@ private async Task SendActivity(Activity activity)
                 {
                     Properties = JObject.FromObject(new {
                         streamId = ((StreamingChannelData) activity.ChannelData).streamId,
-                        streamType = ((StreamingChannelData) activity.ChannelData).StreamType,
+                        streamType = ((StreamingChannelData) activity.ChannelData).StreamType.ToString(),
                         streamSequence = ((StreamingChannelData) activity.ChannelData).StreamSequence,
 
                     })

dotnet/packages/Microsoft.TeamsAI/Microsoft.TeamsAI/Microsoft.Teams.AI.csproj

@@ -7,7 +7,7 @@
     <Nullable>enable</Nullable>
     <PackageId>Microsoft.Teams.AI</PackageId>
     <Product>Microsoft Teams AI SDK</Product>
-    <Version>1.8.0</Version>
+    <Version>1.8.1</Version>
     <Authors>Microsoft</Authors>
     <Company>Microsoft</Company>
     <Copyright>© Microsoft Corporation. All rights reserved.</Copyright>