Moves the standalone vulnerabilities url to `/vulnerabilities/[id]`"

What does this MR do?

This MR moves the standalone vulnerabilities from security/dashboards/[id] to security/vulnerabilities/[id]

Screenshots

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

Availability and Testing

The tests have been kept the same, I've just moved the relevant parts to the relevant specs.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

touches #199222 (closed)