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/rpc/src/peer.rs

@@ -25,7 +25,7 @@ use std::{
 };
 use tracing::instrument;
 
-#[derive(Clone, Copy, Default, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, Serialize)]
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, Serialize)]
 pub struct ConnectionId {
     pub owner_id: u32,
     pub id: u32,