Add `globalSynchronizerHealth` alerts (#6969)

See https://github.com/DACH-NY/canton-network-internal/issues/6371

Regular alerts for now, plan is to promote at least some of them to sev-2 and/or sev-1 after some testing.

- Discarded requests / replays <- also adds this as panels on the Global Synchronizer Utilization dashboard
- Failed confirmation requests
- TPS drop