Improve incident timeline typography

What does this MR do and why?

Part of the general UI tidy of the incident timeline. See issue: #358637 (closed) #384236 (closed)

This MR is specifically for the following item:

1. • Update text size/colors of timeline items (see Figma):

Related discussion here: #358637 (comment 1031512162)

Note: Visual changes only so no test changes required.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

before	after

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Open a project.
2. Navigate to Monitor -> Incidents in the sidebar.
3. Select an existing incident or create a new one.
4. Navigate to the Timeline tab.
5. View existing items or add new ones.

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.