Update removal milestone in deprecation warning

What does this MR do?

Updates the removal milestone in Registering runners so that it's aligned with the removal schedule.

Related issues

gitlab!108963 (comment 1240561673)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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