Add `Start a trial` to Duo Pro usage tab and trackings

What does this MR do and why?

Related to #454602

• Add Start a trial CTA to GitLab Duo Pro tab on /usage_quotas page that directs users to /-/trials/duo_pro/new
• Update styling of the contact sales CTA
• Add trackings to the tab
• Update the marketing link for Duo Pro

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

How to set up and validate locally

• Setup GDK to simulate SaaS.
• Create a new group
• Set the group to have one of these plans: Premium, Ultimate, or Ultimate trial, from /admin/groups/<group_name>/edit as Admin.
• Go to /groups/<group_name>/-/usage_quotas#code-suggestions-usage-tab and verify changes