Add alert_events param to permitted

What does this MR do?

This adds alert_events to the permitted params, so that we can turn on & off the Alert setting in Slack Notifications service.

This was missed in !33017 (merged)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #216326 (closed)