Use dotnet 8 csharp project

Author: kamperiadis

eng/ci/templates/jobs/build.yml

@@ -56,5 +56,5 @@ jobs:
     - task: DotNetCoreCLI@2
       inputs:
         command: 'build'
-        projects: 'tests/e2e/dotnet3/*.csproj'
+        projects: 'tests/e2e/dotnet8/*.csproj'
       displayName: 'Build project for csharp language'