Alert assignee feature flag removal

What does this MR do?

A tiny MVC to close: #218409 (closed)

This merge serves as the UX review for alert assignees dropdown / system notes and the feature flag removal from both the frontend and backend .

This merges comes after: !33217 (merged) being merged first.

This issue closes 4 %13.1 ~Deliverables for ~"Category:Alert Management" in the Assign Alerts Epic

In this merge the listed UX and frontend improvements include:

• Adding tooltips to the system notes
• Highlighting the current user
• Better consistency with the current assignee dropdown
• Mobile spacing
• Mobile header spacing
• Removing the feature flag from the frontend

In this merge the backend changes include:

• Stop N+1 queries on notes by pre-loading
• Order notes by freshness
• Remove backend flag

Screenshots

XL	MD	SM	XS

Notes for testing

Enable Feature.enable(:graphql_lookahead_support) otherwise you experience N+1 queries for assignees.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required) !33069 (merged) !34446 (merged) !34447 (merged)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team