Convert alerts to use task manager intervals

[mikecote]

This portion will require #45152. Once completed, alerting should use the task manager's interval property as well as the update API whenever the interval changes.

[elasticmachine]

Pinging @elastic/kibana-stack-services

[mikecote]

This will require an update API in task manager in order to update interval value (causing a re-schedule). This is getting more similar to #45152 the more I dig into it. Both issues could be resolved in 1-2 PRs. cc @gmmorris.

[mikecote]

Updating issue: This portion will require #45152. Once completed, alerting should use the task manager's interval property as well as the update API whenever the interval changes.

[gmmorris]

I've began investigating the changes needed in alerting and a couple of questions spring to mind:

1. Do we still want to store interval on Alert SavedObjects? If interval is used to schedule the Task, then, in theory, we have no need to store the interval on the Alert, but before I clean off that field I want to make sure this is the case. UPDATE: I have now realised we do need it, as enabling/disabling an Alert deletes the Task, but you don't need to specify the interval when enabling, so we'd need to store the interval somewhere as a source of truth.
2. How do we reschedule existing Alerts once we've made this change? This is a unique migration as it's not a Alert -> Alert migration, but rather, we want to migrate an Alerting Task based on a version change in such a way that info that currently resides on the Alert is used to migrate the Task. As far as I'm aware, migrations can't currently do this.
   Any ideas?

[mikecote]

This is a unique migration as it's not a Alert -> Alert migration, but rather, we want to migrate an Alerting Task based on a version change in such a way that info that currently resides on the Alert is used to migrate the Task.

Luckily we don't need to support migrations yet (ex: 7.5 -> 7.6) as we currently don't support existing alerts in pre 7.6 releases.

[gmmorris]

Ahh so we're fine with just breaking existing alerts when moving form 7.5 to 7.6?
I guess, as we're not even in beta yet that is reasonable.... lucky 😬