Resolve "Refactor re-used code in ee/lib/api/vulnerabilities.rb"

What does this MR do?

This MR reduces the amount of reused code in ee/lib/api/vulnerabilities.rb. Many of the methods are very similar and can benefit from some cleanup. See #214085 (closed).

This utilizes the before and after blocks of the Grape API

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers