Forks remain linked when changing visibility

Problem to solve

Currently when we decrease the visibility of a fork, we unlink the project from the fork network. This is discussed in the issue which unlinked the fork originally #33358 (closed) and was done to fix the issue where the visibility was forced onto the children forks.

Further info

Context in epic: &4405 (closed)

Context in original issue: #241804 (closed)

Proposal

Based on Pedro's suggestion !41103 (comment 419647082), for this issue we stop breaking the relationship and instead enforce the children are not more visible than the parent.

Ie. we need to see if anything is violating the rule of being more visible than its parent, or less visible than its children, then break away from those

Acceptance Criteria:

We can now make a merge request from a confidential issue

The confirmation box warning is changed to be more relevant when unforking