Draft: Remove migrate_user_mentions feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Removes migrate_user_mentions feature flag

Relates to #231175 (closed)

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 #231175 (closed)