Stop unlinking the fork when changing visibility

What does this MR do?

Closes #250681 (closed)

• Remove forks unlinking when the project visibility changes.
• Visibility of forks is not affected
• User can change a fork visibility, but it cannot be higher than original project visibility.
• Change the copy in the confirmation box when changing visibility to remove mention of unlinking
• Remove warning about fork unlinking from documentation pages

Expected behavior

According to #250681 (comment 422724606)

1. Project A is internal.
2. Project A is forked into Project B (both are internal).
3. Project A becomes private. Fork relationships are kept. Project B is still internal, and can only become private, it cannot become public.
4. Project A is forked into Project C (both are private). Project C is private and cannot become internal or public.

Screenshots (strongly suggested)

Setup

• Project A - public
• Project B - public fork of Project A
• Project A becomes private.

Visibility select box of the Project B (fork)

Before click	After click

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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