Fix the seeder 24_forks.rb cannot find public project

What does this MR do?

During the seed_fu process (via rake dev:setup), it creates some project with a random visibility level, as the following code

visibility_level: Gitlab::VisibilityLevel.values.sample,

This could cause an exception in 24_forks.rb which assumes there is at least one public project. If Gitlab::VisibilityLevel.values.sample didn't choose public, then it triggers an error like https://gitlab.com/dosuken123/thin-gdk/-/jobs/138735110

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer