chore: Bump Azure.Identity from 1.19.0 to 1.21.0 (#275)

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


## 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_1.21.0).
</details>

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src/FluentEmail.Graph/FluentEmail.Graph.csproj

@@ -41,7 +41,7 @@ v2.1 Added support for Inline images
     </ItemGroup>
 
     <ItemGroup>
-        <PackageReference Include="Azure.Identity" Version="1.19.0" />
+        <PackageReference Include="Azure.Identity" Version="1.21.0" />
         <PackageReference Include="DotNet.ReproducibleBuilds" Version="2.0.2">
             <PrivateAssets>all</PrivateAssets>
             <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>