Fix Omnibus CTA button height

What does this MR do and why?

Today while testing the new fonts, I learned that pages under /omnibus have a different CTA ("Get free trial") button in the header. I missed this button when switching from Bootstrap to GitLab UI CSS, which means it's currently big and weird on Omnibus pages (like this).

This MR puts the proper CSS classes on the Omnibus button so that it's the same height as the Versions drop-down and consistent across the whole site.

This MR also updates the link to point to the "self-hosted" tab on the marketing site since the existing link no longer did this.

Screenshots, screen recordings, or links to review app

Before	After

Before: https://docs.gitlab.com/omnibus/maintenance/ After: https://sarahg-cta-btn-shenanigans.docs.gitlab-review.app/omnibus/maintenance/

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. View a page under /omnibus and verify the "Get free trial" button in the header is the same height as the version picker.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.