feat: Get rid of MessageState::{OutPreparing,OutMdnRcvd} in the db

`OutPreparing` is deprecated since 2024-12-07, replace it with `OutFailed`, such messages are
probably not interesting anymore. `OutMdnRcvd` is not used in the db for new messages since
a30c6ae1f7, `OutDelivered` is stored instead.

src/webxdc.rs

@@ -550,7 +550,7 @@ impl Context {
 
         let send_now = !matches!(
             instance.state,
-            MessageState::Undefined | MessageState::OutPreparing | MessageState::OutDraft
+            MessageState::Undefined | MessageState::OutDraft
         );
 
         status_update.uid = Some(create_id());