Remove v4 model listing endpoint

[carmenfan]

Description

With #5382, we reinstated the model listing endpoint. We want to remove it again for the upcoming release.

But we want to ensure we're doing the decommissioning approach described here: #5317 to avoid unnecessary support queries.

Goals

• As a 3DR admin, I want to decommission endpoints that should no longer be used
• As a user, I want an appropriate error message if i called a decomissioned endpoint

Tasks

To be filled in by the developer