Default prevent_visibility_restriction on and update docs

Turn the FF prevent_visibility_restriction on by default.

The following discussion from !124649 (merged) should be addressed:

• !124649 (comment 1478227373).

  When we default enable, or remove the feature flag, let's make sure to update https://docs.gitlab.com/ee/administration/settings/visibility_and_access_controls.html#restrict-visibility-levels to note admins cannot restrict a default visibility level.

• !124649 (comment 1459218728).

Consider updating docs, adding helper text under Default group visibility and modifying validation messages to better explain how Default group visiblity, Default project visibility and Restricted visibility levels can conflict.

Suggested changes: !124649 (comment 1460641449)