chore(csharp): Bump Azure.Identity from 1.17.1 to 1.21.0

---
updated-dependencies:
- dependency-name: Azure.Identity
  dependency-version: 1.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

csharp/Directory.Packages.props

@@ -28,7 +28,7 @@
     <PackageVersion Include="Apache.Arrow.Adbc.Drivers.Interop.Snowflake" Version="[$(Version)]" />
     <PackageVersion Include="Apache.Arrow.Flight" Version="22.1.0" />
     <PackageVersion Include="ApacheThrift" Version="0.22.0" />
-    <PackageVersion Include="Azure.Identity" Version="1.17.1" />
+    <PackageVersion Include="Azure.Identity" Version="1.21.0" />
     <PackageVersion Include="BenchmarkDotNet" Version="0.15.8" />
     <PackageVersion Include="coverlet.collector" Version="8.0.1" />
     <PackageVersion Include="DuckDB.NET.Bindings.Full" Version="1.4.4" />