Convert OAuth2 authorization panel to GitLab UI utility classes

What does this MR do and why?

Related to #344556 (closed)

Removes modal-* CSS classes from OAuth2 authorization panel and replaces them with GitLab UI utility classes.

Screenshots or screen recordings

Desktop

Before	After

Mobile

Before	After

How to set up and validate locally

Testing the full OAuth2 flow is a bit tricky locally because you need to setup an OAuth2 provider. In this case we don't need to test the full flow since we are just changing the styling of the authorize panel so we can follow the instructions below:

1. Login to GitLab as an Admin and navigate to /admin/applications/new
2. Fill out the form with the following:
   • Name - Foo bar
   • Redirect URI - http://foo.bar
   • Check api under Scopes
3. Navigate to /oauth/authorize?access_type=online&client_id=<Application ID>&redirect_uri=http%3A%2F%2Ffoo.bar&response_type=code&scope=api. Make sure you update <Application ID> to the Application ID provided in the previous step

MR acceptance checklist

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