[C#] chore: Bump to preview-6 (#1040)

## Linked issues

closes: #989  (issue number)

## Details

Bump to `preview-6`

## 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 (we use
[TypeDoc](https://typedoc.org/) to document our code)
- 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

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>1.0.0-preview-5</Version>
+    <Version>1.0.0-preview-6</Version>
     <Authors>Microsoft</Authors>
     <Company>Microsoft</Company>
     <Copyright>© Microsoft Corporation. All rights reserved.</Copyright>