Update styling of /welcome page to match other areas

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What does this MR do and why?

Fixes: https://gitlab.com/gitlab-org/gitlab/-/issues/527134/

This MR updates the styling of the welcome page. This affects two different flows for entry:

1. A new user signs up (free user)
2. A user signs up via trial.

EE: true

Changelog: changed

References

Screenshots or screen recordings

Trial Signup

Current /welcome page	Updated /welcome page

Free User Signup

Current /welcome page	Updated /welcome page

How to set up and validate locally

For new (free) user setup:

1. Setup to simulate SaaS and restart GDK
   • Make sure Admin > Settings > General > Sign Up Restrictions match the below (no admin approval, but hard email confirmation)
2. Register as a new user.
3. Confirm email by finding the confirmation path from last User created in rails console
   • Rails.application.routes.url_helpers.user_confirmation_path(confirmation_token: User.last.confirmation_token)
4. Visit the path from the previous step.
5. You should be on the sign in page, log back in.
6. You should see the screen with the changes

For trial setup:

• Setup to simulate SaaS and restart GDK
• Access this link: here
• Continue with the signup setup
• You should see the screen with the changes after signing up.

MR acceptance checklist

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