Update MSRV to 1.84.0

And appease clippy (`__CARGO_FIX_YOLO=1 cargo clippy --fix` plus some
manual type shuffling).

src/api/server_server.rs

@@ -1030,8 +1030,8 @@ pub(crate) async fn send_transaction_message_route(
                     services().users.add_cross_signing_keys(
                         &user_id,
                         &master_key,
-                        &self_signing_key,
-                        &None,
+                        self_signing_key.as_ref(),
+                        None,
                         true,
                     )?;
                 }