Exited Dashboardv2 Validators Trigger Alerts

[nhpurpose]

Describe the bug
Validators tracked in the v2 dashboard send offline validator webhook alerts when the validator has exited.

To Reproduce
Steps to reproduce the behavior:

1. Add validator to dashboard
2. Enable webhook alerts
3. Exit validator

Expected behavior
Exited validators are typically exited on purpose, so at most a single alert "validator X has exited" rather than alerting that the validator is offline every epoch. One should remove exited validtaors from the active set when exiting them, but this is still inaccurate alerting behavior.