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

What does this MR do and why?

Migrate form_errors in app/views/shared/web_hooks/_form.html.haml to Pajamas::AlertComponent.

Screenshots or screen recordings

Scenario	Screenshot
Before
After

How to set up and validate locally

1. Go to any project on your local GDK.
2. In the left menu bar go to - Settings -> Webhooks.
3. Add a new webhook with any values.
4. Then click on the edit button against the newly added webhook. (The newly added webhook will be shown below the form.)
5. Change the URL to blank and click Save changes.
6. 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 #364948 (closed)