DRY up a few hard-coded URLs to the Marketing site

The following discussion from !76476 (merged) should be addressed:

• #370351+

  suggestion (non-blocking): I've notice that https://about.gitlab.com/ address is repeated in few places and even https://about.gitlab.com/free-trial is already present in codebase. I wonder if that would make sens to extract a class like https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/subscription_portal.rb and abstract all those hardcoded links in one place. WDYT? Since this seems to be outside of this MR scope, I'm ok with this being resolved as follow up.