Secret detection vulnerabilities not auto set to "No longer detected"

What does this MR do?

This MR adds some explanation on why secret detection vulnerabilities are not automatically set to "No longer detected", when secrets are eliminated from the scanned files. This came up in a ticket and in the follow-up.

@connorgilbert explained it beautifully in an internal thread:

We don’t mark Secret Detection results as “no longer detected”, because they can’t be fixed by simply changing the default branch. (This is different from the other vuln types—if you update your code to not be vulnerable to a SAST problem, it’s now fixed; same for a vulnerable dependency that you update.) Secrets are once-leaked, always-leaked. So the intended action is to review the report, then do whatever you need to do to remediate the leak (which is usually to rotate the credential that was leaked).

My intention is to clarify this in the docs as well.

Related issues

#431712 (comment 1681043497)

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