Add discussions link to GitHub issues template (#23223)

.github/ISSUE_TEMPLATE/config.yml

 blank_issues_enabled: true
 contact_links:
+  - name: Discussions
+    url: https://github.com/dotnet/aspnetcore/discussions/new
+    about: Is your issue perhaps more of a discussion? Please open topics for discussion using the new GitHub Discussions feature.
   - name: Issue with .NET runtime or core .NET libraries
     url:  https://github.com/dotnet/runtime/issues/new/choose
     about: Please open issues relating to the .NET runtime or core .NET libraries in dotnet/runtime.