Update GitLab version in DELETE REST API docs for protected containers

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA

What does this MR do and why?

The DELETE endpoint of the REST API of protected containers will be added with milestone 17.4, not 17.3. Therefore, this commit updates the milestone.

Issue: #457518 (closed)

MR introducing the endpoint: !159619 (merged)

🛠 with ❤ at Siemens

MR acceptance checklist

MR Checklist ( @nwittstruck)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

Related to #457518 (closed)