[Backport] Exempt unicorn['svlogd_prefix'] from deprecation check

What does this MR do?

13.12.x backport of !5395 (merged)

Related issues

#6222 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
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• [-] trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• [-] Test plan indicating conditions for success has been posted and passes
• [-] Documentation created/updated
• Tests added
• [-] Integration tests added to GitLab QA
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