Update paid plan links

Why is this change being made?

We're working on a new paid signup flow for GitLab.com. This MR changes the link from the pricing page to the new subscription page. The MR can be merged when gitlab-org/gitlab!22978 (merged) is deployed since it will still redirect to the customer portal unless the feature flag is enabled.

UPDATE: gitlab-org/gitlab!22978 (merged) is deployed. https://gitlab.com/-/subscriptions/new redirects to customer portal now until the feature flag is enabled.

Closes gitlab-org/growth/engineering#47 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what