[Alerting] Update alerts client audit events to new terminology

[ymao1]

Currently, the audit messages generated by the alerts client all use some form of alert_. With this PR, the RAC effort will be adding an alerts client for accessing alerts as data an also generating audit events with the alert_ prefix. There was a question of whether the audit events needs to match the saved object type (which for alerting is currently still alert) but based on this comment, this doesn't seem to be an issue, so we should update our audit events to use rule_ to be consistent with the new terminology and to avoid confusion.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)