Update Discover Duo Pro for smaller screens

What does this MR do and why?

• Fix images overlaps
• Fix grid alignment
• Align image border-radius with card border-radius
• Use gap instead of different margin for different breakpoints
• Remove unnecessary grid for buttons

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a new group
2. Run bin/rails c

   g = Group.last
   u = User.first
   g.owner = u
   g.save!
   GitlabSubscriptions::AddOn.find_or_create_by(name: 'code_suggestions', description: GitlabSubscriptions::AddOn.descriptions[:code_suggestions])
   GitlabSubscription.last.update(trial: true, trial_starts_on: 5.days.ago, trial_ends_on: Date.tomorrow)
   GitlabSubscriptions::AddOnPurchase.create(subscription_add_on_id: GitlabSubscriptions::AddOn.find_by(name: 'code_suggestions').id, namespace: Group.last, started_at: 5.days.ago, expires_on: Date.tomorrow, trial: true, quantity: 1, purchase_xid: 1,  organization_id: 1)

3. Go to http://gdk.test:3000/groups/YOUR_GROUP_NAME/-/add_ons/discover_duo_pro
4. Change the window size

#474922 (closed)