Retry letter topic shouldn't be supported for non shared sub.

As doc says: `Currently, retry letter topic is enabled in Shared subscription types.`
But in fact, all sub type support retry topic. I think retry topic shouldn't be supported for non shared sub, because of message order considieration.

Start with the retry letter topic documentation and the subscription-type behavior described in this issue, then trace how retry topics are currently accepted for each subscription type. Done means non-shared subscriptions no longer support retry topics, shared subscriptions retain their documented support, and message-ordering implications are addressed.

Written by the indexing model from the issue text.