Updates package request forwarding settings description

What does this MR do and why?

Update description related to security risks and adds link to documentation for package request forwarding settings

Note: maven settings is under a feature flag, so you might not see related checkboxes if it is not enabled on your local GDK.

This is the part 1 of 2 MRs for https://gitlab.com/gitlab-org/gitlab/-/issues/388694+ where the link to documentation is added for admin & group settings

Part 2 will be adding the text to delete modals in package registry

Screenshots or screen recordings

Admin settings

Group settings

How to set up and validate locally

1. Visit /admin/application_settings/ci_cd
2. Make sure new description is visible.
3. Enable maven_central_request_forwarding & make sure maven settings are also visible.
4. Visit group Settings > Packages & registry settings.
5. Make sure new section for settings is visible. Enable maven_central_request_forwarding, make sure maven settings are also visible.
6. Make sure new description is visible.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #388694