Migrate `form_errors` in GPG keys to Pajamas alert

What does this MR do and why?

Migrate form_errors in app/views/profiles/gpg_keys/_form.html.haml to Pajamas::AlertComponent.

Screenshots or screen recordings

Scenario	Screenshot
Before
After

How to set up and validate locally

1. Click on the user avatar in the top right menu.
2. Click on Edit profile.
3. In the left menu go to GPG keys.
4. Enter an invalid key like abc.\
5. Click Add key.
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 #364955 (closed)