Refine UI for the dependency proxy settings group level

🔥 Problem

With Maven dependency proxy (&3610 - closed), we're adding the dependency proxy for packages.

It will work at the project level. As such, it will have settings at the project level.

The problem is that we have the dependency proxy for container repositories at the group level: https://docs.gitlab.com/ee/user/packages/dependency_proxy/ and here is the UI:

It is not clear that these settings apply to the dependency proxy for container repositories and not to the dependency proxy for packages.

🚒 Solution

Clarify the UI that the dependency proxy settings at the group level are for the dependency proxy for container repositories.

⌨ Implementation guide

Change the text to Enable the Dependency Proxy to cache container images from Docker Hub and auto-clear the cache.