Rust 1.78 (#11314)

Notable things I've had to fix due to 1.78:
- Better detection of unused items
- New clippy lint (`assigning_clones`) that points out places where assignment operations with clone rhs could be replaced with more performant `clone_into`
Release Notes:

- N/A

crates/collab/src/api.rs

@@ -116,13 +116,6 @@ struct CreateUserParams {
     invite_count: i32,
 }
 
-#[derive(Serialize, Debug)]
-struct CreateUserResponse {
-    user: User,
-    signup_device_id: Option<String>,
-    metrics_id: String,
-}
-
 async fn get_rpc_server_snapshot(
     Extension(rpc_server): Extension<Option<Arc<rpc::Server>>>,
 ) -> Result<ErasedJson> {