Rename incident_timeline_event_tab FF to incident_timeline

What does this MR do and why?

Renames incident_timeline_event_tab feature flag to incident_timeline.

We have two different rollout issues (one, two) for two different flags. We'd like to merge two feature flags into one and use it for the whole feature.

Here is the initial discussion with more information about that decision !82596 (comment 873066936)

The renaming is safe, the feature flag has not been enabled yet.

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