[C#] fix: Change bracket notation to object initializer (#1661)

## Linked issues

closes: #minor (issue number)

## Details
- Change bracket notation (C#12 feature) to object initializer since it
is not supported in .NET 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 (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

dotnet/packages/Microsoft.TeamsAI/Microsoft.TeamsAI/Utilities/CitationUtils.cs

@@ -51,7 +51,7 @@ public static string FormatCitationsResponse(string text)
             }
             else
             {
-                List<ClientCitation> usedCitations = [];
+                List<ClientCitation> usedCitations = new();
                 foreach (Match match in matches)
                 {
                     citations.Find((citation) =>