Replace legacy css for Duo Pro access denied page

What does this MR do and why?

Replacing legacy CSS classes for ee/app/views/subscriptions/trials/duo_pro/access_denied.html.haml:

gl--flex-center
gl-flex-direction-column
gl-sm-flex-direction-row
gl-sm-w-auto

MR acceptance checklist

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Related to #463075 (closed)