Fix for group transfer service to deny transferring a group to its subgroup

What does this MR do?

Attempt to transfer a group to one of its sub-group results in an exception:

Errno::EINVAL (Invalid argument @ rb_file_s_rename - (/opt/gitlab/embedded/service/gitlab-rails/public/uploads/group-1, /opt/gitlab/embedded/service/gi
tlab-rails/public/uploads/group-1/sub-group-1/group-1))

The issue occurs because as part of the transfer, group directories are moved to its new parent directory. In case of attempt to transfer a group to one of its sub-group, this move is not possible at filesystem level hence FileUtils.move raises exception.

The issue can be reproduced by following steps:

1. Create a group - Group-1
2. Create a sub-group under Group-1 named Group-2
3. Transfer Group-1 to Group-2

This MR fixes the bug by introducing a constraint preventing transfer of a group to one of it's sub-group.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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