Update dependency Azure.Identity to 1.21.0 (#5407)

Co-authored-by: dependencyupdates[bot] <218638057+dependencyupdates[bot]@users.noreply.github.com>

Author: dependencyupdates[bot]

src/Directory.Packages.props

@@ -6,7 +6,7 @@
   <ItemGroup Label="Versions for direct package references">
     <PackageVersion Include="Autofac" Version="9.0.0" />
     <PackageVersion Include="AWSSDK.CloudWatch" Version="4.0.7" />
-    <PackageVersion Include="Azure.Identity" Version="1.19.0" />
+    <PackageVersion Include="Azure.Identity" Version="1.21.0" />
     <PackageVersion Include="Azure.Monitor.Query.Metrics" Version="1.0.0" />
     <PackageVersion Include="Azure.ResourceManager.ServiceBus" Version="1.1.0" />
     <PackageVersion Include="ByteSize" Version="2.1.2" />