Update fork network relations upon project visibility decrease

What does this MR do?

When decreasing the visibility level of projects, instead of also changing its forks visibility we instead remove itself (the project) from the fork network, also unlinking its direct forks (but not deleting any membership).

A feature flag was added in order to fall back to the old behavior of updating the forks.

Screenshots

Now, when a project is deleted, or unlinked from its forks (now through this visibility level decrease), its direct forks will present the following in the home page:

#33358 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content