doc: Remove outdated flag from secret post_processing.md
What does this MR do and why?
The previous rebase of !109700 (comment 1250074363) missed this removal
How to set up and validate locally
Numbered steps to set up and validate the change are strongly suggested.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.