Removes `undefined` from security scanner docs

What does this MR do?

Removes the (hopefully) last few references to the undefined severity and/or confidence value from our documentation.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• Separation of EE specific content

Closes #202685 (closed)