Support restricted visibility level in fork form

What does this MR do?

This MR adds support for "restricted visibility level"

restricted visibility level 👇	Private	Internal	Public
Private (0)	cell
Internal (10)	cell
Public (20)	cell

NOTE!

There are instances when all visibility levels would be disabled, rendering no visibility can be selected. For example:

Project = private | Namespace = private | restrictedVisibilityLevel = private (0)

In this case, I'm going to ignore the restrictedVisibilityLevel and set the visibility level to be "Private"

Testing Case

This feature is behind a Feature Flag, to turn on:

Feature.enable(:fork_project_form)

To set the "restricted visibility level", you need to navigate to the admin panel:

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not 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.

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

Related to #329575 (closed)