Updated premium support subscription links

@rdavila @ChadMalchow I've updated the support links to point to the new payment app.

On https://about.gitlab.com/gitlab-com/ I linked the `Premium Support` bullet point to the pricing page — to me that was more logical to learn more versus linking to the Sign In page on the payment app.
I also linked to the pricing page for the `Premium Support` links on https://about.gitlab.com/getting-help/ for the same reason.
@iamphill is it possible to create something like `about.gitlab.com/pricing/#options` so that we can link to and show that exact section, similar to how the handbook works?
cc @AshleyS @amara to checkout my copy updates