Update dependency NUnit to v4

Author: dependencyupdates[bot]

src/NServiceBus.Envelope.CloudEvents.Tests/NServiceBus.Envelope.CloudEvents.Tests.csproj

@@ -17,7 +17,7 @@
   <ItemGroup>
     <PackageReference Include="GitHubActionsTestLogger" Version="2.4.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
-    <PackageReference Include="NUnit" Version="3.14.0" />
+    <PackageReference Include="NUnit" Version="4.4.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.6.0" />
   </ItemGroup>