Bump Azure.Identity from 1.20.0 to 1.21.0 (#979)

Updated [Azure.Identity](https://github.com/Azure/azure-sdk-for-net)
from 1.20.0 to 1.21.0.

<details>
<summary>Release notes</summary>

_Sourced from [Azure.Identity's
releases](https://github.com/Azure/azure-sdk-for-net/releases)._

## 1.21.0

## 1.21.0 (2026-04-10)

### Other Changes

- All `Azure.Identity` types have been moved to `Azure.Core` and are now
available through `TypeForwardedTo` attributes. This is a non-breaking
change — existing code continues to work transparently. The library's
version number now aligns with that of `Azure.Core`. See the [Migration
Guide](https://github.com/Azure/azure-sdk-for-net/blob/b1dfb1ec786d4013cff05fd10436c069ecb37304/sdk/identity/Azure.Identity/MigrationGuide.md)
for details.


Commits viewable in [compare
view](Azure/azure-sdk-for-net@Azure.Identity_1.20.0...Azure.Identity_1.21.0).
</details>

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Directory.Packages.props

@@ -21,7 +21,7 @@
   <ItemGroup>
     <PackageVersion Include="AspNet.Security.OAuth.GitHub" Version="10.0.0" />
     <PackageVersion Include="Azure.Extensions.AspNetCore.Configuration.Secrets" Version="1.5.0" />
-    <PackageVersion Include="Azure.Identity" Version="1.20.0" />
+    <PackageVersion Include="Azure.Identity" Version="1.21.0" />
     <PackageVersion Include="Azure.Monitor.OpenTelemetry.AspNetCore" Version="1.4.0" />
     <PackageVersion Include="Microsoft.ApplicationInsights.Profiler.AspNetCore" Version="3.0.2" />
     <PackageVersion Include="TUnit" Version="1.30.8" />