Allow users to delete incident timeline events

What does this MR do and why?

Allow users to delete incident timeline events

Based on !81284 (closed), split into 3 MRs

Issue	MR
Display Incident Timeline Events (#344057 - closed)	!89210 (merged)
Add timeline event manually (#344059 - closed)	!89211 (merged)
Delete timeline event (#344061 - closed)	!89212 (merged)	<--- You are here
[Feature flag] Rollout of `incident_timeline` (#353426 - closed)	!92169 (closed)

Issue: #344061 (closed)

Screenshots or screen recordings

before	after

delete dropdown

How to set up and validate locally

Prerequisite: enable the :incident_timeline feature flag, and ensure you are logged in as a Reporter or above.

1. Navigate to the incident list.
   • Monitor -> Incidents.
2. Create an incident.
3. Navigate to the timeline tab.
4. Create a new timeline item.
5. Delete the timeline item.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.