Bump Azure.Messaging.EventGrid from 4.20.0 to 4.21.0 in /src (#174)

Bumps [Azure.Messaging.EventGrid](https://github.com/Azure/azure-sdk-for-net) from 4.20.0 to 4.21.0.
- [Release notes](https://github.com/Azure/azure-sdk-for-net/releases)
- [Commits](Azure/azure-sdk-for-net@Azure.Messaging.EventGrid_4.20.0...Azure.Messaging.EventGrid_4.21.0)

---
updated-dependencies:
- dependency-name: Azure.Messaging.EventGrid
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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src/AzureEventGridSimulator.Tests/AzureEventGridSimulator.Tests.csproj

@@ -7,7 +7,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Azure.Messaging.EventGrid" Version="4.20.0" />
+    <PackageReference Include="Azure.Messaging.EventGrid" Version="4.21.0" />
     <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="7.0.12" />
     <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.7.2" />