Remove pages_serve_from_migrated_zip feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

We already tested this feature flag on gitlab.com and migrated data looked fine.

In %13.11 we also introduce the background migration, so we still need some kind of workaround if something goes wrong on the self-hosted environment.

But we actually have a rake task for rolling back the migration(see !59003 (diffs)), and I think it's enough.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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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
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Security

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Related to #300021 (closed)