Alerting: Remove UALERT_MIG env guard from dashboard rule migration (#34384)
Alerting: Remove UALERT_MIG env guard from dashboard rule migration (#34384)

Rules/notifications/etc migration will now be activated with feature flag alone.
When the feature flag is enabled dashboard alerts are migrated into the system.
When the feature flag is removed, all migrated and newly created alerts in the new system are deleted.
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