Update dependency proxy to ultimate

Why is this change being made?

As I was setting up the dependency proxy in GDK, I realized that an ultimate license is required. Features.yml needs to be updated to reflect that as well as the documentation.

This MR: gitlab-org/gitlab!23925 (merged) updates the admin and user documentation accordingly

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