Update tier for the Dependency Proxy to premium

Why is this change being made?

The Dependency Proxy is available as a premium feature. In the features.yml we had it listed as Ultimate only. This MR corrects that and updates the tiers to reflect what is actually available.

How will this merge request impact page speed?

no

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

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