panic: cannot drop instances with installed collections

[sentry-io]

Sentry Issue: DATABASE-BACKEND-3MV

panic: cannot drop instances with installed collections
  File "panicking.rs", line 652, in rust_begin_unwind
  File "instance.rs", in mz_compute_client::controller::instance::Instance<T>::drop
  File "controller.rs", line 586, in mz_compute_client::controller::ComputeController<T>::drop_instance
  File "clusters.rs", line 370, in mz_controller::Controller::drop_cluster
  File "ddl.rs", line 655, in mz_adapter::coord::Coordinator::catalog_transact_inner::{closure#0}::{closure#0}::{closure#9}
...
(60 additional frame(s) were not displayed)

[teskje]

Context in Slack.

This is likely related to blue/green deployments. My suspicion is that something about ALTER {SCHEMA,CLUSTER} ... SWAP confuses the coordinator enough that it forgets about dataflows it previously installed on the swapped clusters.