Migrate `form_errors` in _gitpod.html.haml to Pajamas alert

What does this MR do and why?

Migrate form_errors in app/views/admin/application_settings/_gitpod.html.haml to Pajamas::AlertComponent.

Screenshots or screen recordings

Scenario	Screenshot
Before
After

How to set up and validate locally

1. Login to your local GDK as an admin user.
2. In admin area go to Settings -> General
3. Expand the Gitpod section.
4. Enable the checkbox Enable Gitpod integration.
5. Do not enter Gitpod URL.
6. Click Save changes.
7. 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 #364885 (closed)