Enable :incident_timeline feature flag by default

What does this MR do and why?

Enables the :incident_timeline feature by default.

Also incorporates some frontend changes, based on pre-release UX feedback.

Important notes:

• This is was prefixed with Draft: because the feature flag rollout steps are were still underway. **Update: **The rollout to gitlab.com production is now complete, so this MR is now ready to go 🚀.
• The MR is code-complete.
• The docs MR has been merged.

See the rollout issue.

Screenshots or screen recordings

before	after

How to set up and validate locally

Prerequisite: Unset the :incident_timeline feature flag if it is already set.

1. Navigate to a project.
2. Navigate to the incident list.
   • Monitor -> Incidents in the left menu.
3. Create a new incident.
4. Navigate to the Timeline tab.

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.