This PR migrates the vast majority of Alerting legacy code to the Kibana Platform.
This includes:
1. Removed legacy Task Manager
2. Migrates Fixture plugins in Alerting, Triggers UI and Task Manager Perf
This does not includes:
1. The PagerDuty simulator due to a lack of support for custom responses in the platform. issue opened. https://github.com/elastic/kibana/issues/65045
2. The Webhooks simulator due to a lack of support for custom authorisation. Requires investigation.