Resolve "Data and Privacy Agreement for GitLab users"

This was originally implemented in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30808, but caused test failures in EE and is now correctly targeting EE with necessary CE backporting.

What does this MR do?

Add notification for an updated privacy policy.

The callout name for the privacy policy update is tagged with the issue number in case we have subsequent privacy policy updates which may warrant new notifications in the future.

This also fixes some flaky tests in ee/spec/support/shared_examples/gold_trial_callout_shared_examples.rb.

Screenshots

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Testing instructions

Remove the Gitlab.com? check in app/views/layouts/_page.html.haml, start your GDK, and view any page to see the banner!

One can remove the callout dismissal record via psql.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Performance 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

Closes #13665 (closed)