Alerts migration cleanup #60356
Description
In order to get alerts migration through, we added a lot of temporary code. This issue aims to remove all of that
- chore(alerts): migration UI cleanup #61347
- chore(alerts): remove migration email templates #62461
- https://github.com/getsentry/getsentry/pull/12307
- (Backend gs) remove migration email script
- Remove related feature flags
- Add custom metric alert type in alert list #60574
- Drop table of migration
- Remove redash dashboards/queries #85438