Fix docs about artifacts:public status on .com

What does this MR do?

The feature flag for artifacts:public, non_public_artifacts, is not enabled on GitLab.com (and never has been), but the docs stated that it was. The rollout was delayed to simplify the implementation of the changes being recommended in https://gitlab.com/gitlab-org/gitlab/-/issues/322454 (migrating data in CI related tables on GitLab.com is very difficult due to their large size).

Rollout issue: #294503 (closed)

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.