Use correct migration finalisation method

What does this MR do and why?

Describe in detail what merge request is being backported and why

This MR backports !119315 (merged) to %15.11, so that customers coming from GitLab 15 have their migrations finalised before moving to %16.0.

Closes #376709 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

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If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
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