Migrate `form_errors` in milestones/_form.html.haml to Pajamas alert

What does this MR do and why?

Migrate form_errors in app/views/projects/milestones/_form.html.haml to Pajamas::AlertComponent.

Screenshots or screen recordings

Scenario	Screenshot
Before
After

How to set up and validate locally

1. On your local GDK, go to any project eg. http://localhost:3000/flightjs/Flight
2. From the left menu bar, go to Issues -> Milestones.
3. Click on New milestone button.
4. Create a milestone such that Due date is before Start date.
5. Observe that alert is shown.

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 #364886 (closed)