Update wording for sast fp banner

What does this MR do and why?

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This change updates the text description for GitLab Duo's false positive detection feature in the security dashboard. The main modification clarifies that the system will review "new" vulnerabilities going forward, rather than implying it reviews all existing vulnerabilities.

The updated message better communicates to users that the feature will automatically analyze future security findings, not retroactively process old ones and additionally links to relevant documentation.

Refs:

Update False Positive Detection Banner with Dis... (#583894) • SAM FIGUEROA • 18.8

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