chore: Bump Azure.Identity from 1.19.0 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]

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>