Prevent Transfering a Group with a Subscription

What does this MR do?

Prevents transferring a group with a subscription to another group.

It is currently possible to transfer a root group with a subscription to another group. The group becomes a subgroup with an associated subscription. This should not be possible; only root groups may have a subscription. This MR fixes the issue by preventing the transfer of any group with a subscription.

More details are available in the issue.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/330524

Screenshots

Go to the group page, then go to Settings > General > Advanced. There you will see the Transfer group pane pictured below.

Before

This is how the Transfer group pane looks currently for all groups. It also still looks this way if the group does not have a subscription or for any subgroups.

After

This is how the Transfer group pane looks for a root group with a subscription after this MR.

If the parent group select dropdown is enabled via Chrome Devtools or the like, then the backend still denies the request and displays a flash message like the following.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team