Convert upgrade link experiment to feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Migrates experiment to a default enabled feature flag upgrade_link_in_user_menu_a for one milestone and adds a changelog.

The upgrade link was previously running as an experiment to 50%, and was then increased to display to 100% after the experiment was validated successful.

Related to experiment cleanup issue: https://gitlab.com/gitlab-org/gitlab/-/issues/300824

• Experiment tracking issue
• Original engineering issue

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #300824