Improve UX for 404 error for Duo Pro trial page

What does this MR do and why?

Improve UX for 404 page when non-group owner role attempts to access /-/trials/duo_pro/new

MR acceptance checklist

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Before	After

How to set up and validate locally

• Setup GDK to simulate SaaS.

bin/rails c

Feature.disable(:duo_pro_trials)

• Sign out. Sign up with new user.
• Visit /-/trials/duo_pro/new. Make sure page returns 404.

bin/rails c

Feature.enable(:duo_pro_trials)

• Reload page. Make sure page returns access denied.
• Go back. Create new group.
• Visit /-/trials/duo_pro/new. Make sure page returns access denied.
• In rails console make it ultimate.

bin/rails c

Group.last.gitlab_subscription.update(hosted_plan: Plan.find_by(name: 'ultimate'))

• Go back. Visit group billing page.
• Click Start a free GitLab Duo Pro trial.
• Make sure page loads Duo Pro trial.
• Visit /-/trials/duo_pro/new?namespace_id=1.
• Make sure page returns access denied.
• Click Go back button.
• Make sure browser redirects back.
• Go forward. Click sign out button.
• Make sure you are signed out.

Related to #460504