Maven virtual registry: refine access requirements
## :fire: Problem
Currently, accessing (pulling file through) the Maven virtual registry [requires](https://gitlab.com/gitlab-org/gitlab/-/blob/10dcc5932d5717b625259a316c94d9528ddd02e5/ee/app/policies/virtual_registries/policies/group_policy.rb#L22):
1. be an administrator _or_
1. be a direct members of the top level group (`guest`) _or_
1. be a direct member of one of the included projects.
We would like to remove (3.).
Before doing so, we're going to have a `Transition period`. This issue prepares everything for that.
## :fish: Transition period
~backend tasks:
* Log users that are granted `read_virtual_registry` through rule (3.). Log transparently (eg. the access is still granted) the situation.
~frontend tasks:
* Add an UI Banner in virtual registry pages announcing that (3.) will be removed from the requirements.
Additional tasks:
* Communicate this in the [feedback issue](https://gitlab.com/gitlab-org/gitlab/-/issues/543045).
issue