Add learn duo pro discover link in expired duo pro trial widget

What does this MR do and why?

Add learn duo pro discover link in expired duo pro trial widget

• add link to discover more
• add tracking for that link
• why, see: https://gitlab.com/gitlab-org/gitlab/-/issues/465929

Changelog: other EE: true

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

Screen_Recording_2024-09-03_at_4.22.50_PM

How to set up and validate locally

• Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
• Sign out. Sign up and create new group during sign up or afterwards.
• Seed add on with this command line(doesn't hurt to run if already add on is already present): FILTER=add_ons be rake db:seed_fu

bin/rails c

GitlabSubscription.last.update(trial: true, trial_starts_on: 45.days.ago, trial_ends_on: 15.days.ago)
GitlabSubscriptions::AddOnPurchase.create(subscription_add_on_id: GitlabSubscriptions::AddOn.find_by(name: 'code_suggestions').id, namespace: Group.last, organization_id: 1, started_at: 69.days.ago, expires_on: 9.days.ago, trial: true, quantity: 1, purchase_xid: '1')

• Visit group page. Make sure you can see expired Duo Pro trial widget.

Related to #465929