Remove Default impl for ConnectionId (#7452)

We noticed the following message in my logs when trying to debug some
lag when collaborating:

```
2024-02-06T09:42:09-08:00 [ERROR] error handling message. client_id:3, sender_id:Some(PeerId { owner_id: 327, id: 1123430 }), type:GetCompletions, error:no such connection: 0/0
```

That `0/0` looks like a bogus connection id, constructed via a derived
`Default`. We didn't ever find a code path that would *use* a default
`ConnectionId` and lead to this error, but it did seem like an
improvement to not have a `Default` for that type.

Release Notes:

- N/A

Co-authored-by: Marshall <marshall@zed.dev>

crates/collab/src/rpc.rs

@@ -1691,7 +1691,7 @@ async fn leave_project(request: proto::LeaveProject, session: Session) -> Result
     tracing::info!(
         %project_id,
         host_user_id = %project.host_user_id,
-        host_connection_id = %project.host_connection_id,
+        host_connection_id = ?project.host_connection_id,
         "leave project"
     );