Bump Azure.Identity from 1.19.0 to 1.20.0 (#961)

Updated [Azure.Identity](https://github.com/Azure/azure-sdk-for-net)
from 1.19.0 to 1.20.0.

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

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

## 1.20.0

## 1.20.0 (2026-03-30)

### Features Added

- Added a JSON schema segment to the NuGet package that provides
IntelliSense and validation for Azure.Identity credential configuration
in `appsettings.json`.

### Breaking Changes

- `AddAzureClient`, `AddKeyedAzureClient`, and `WithAzureCredential`
return type changed from `IHostApplicationBuilder` to `IClientBuilder`
to align with the `IClientBuilder` composition change in
System.ClientModel.


Commits viewable in [compare
view](Azure/azure-sdk-for-net@Azure.Identity_1.19.0...Azure.Identity_1.20.0).
</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="1.19.0" />
+    <PackageVersion Include="Azure.Identity" Version="1.20.0" />
     <PackageVersion Include="Azure.Monitor.OpenTelemetry.AspNetCore" Version="1.4.0" />
     <PackageVersion Include="Microsoft.ApplicationInsights.Profiler.AspNetCore" Version="3.0.1" />
     <PackageVersion Include="TUnit" Version="1.23.7" />