Add attachment to disable in toolbar items

What does this MR do and why?

Relates to #373195 (closed)

Disables the attachment icon in the timeline event form on the UI.

Screenshots or screen recordings

Check there is no attachment icon.

How to set up and validate locally

1. Open or create an incident from Monitor>Incidents.
2. Select Timeline tab.
3. Select Add new timeline event button.
4. Check the form and see there is no attach-file icon on the header.

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.

Related to #373195 (closed)