Allow processing multline issues from gosec

What does this MR do?

Gosec can report multiline issues in the format <start_line>-<end_line> as you can see on these test cases. This MR allows the converter function to parse multiline issues reported from gosec. If it cannot convert the string line to an integer, it means that the issue is a multiline one so the line range gets extracted.

What are the relevant issue numbers?

Relates to gitlab-org/gitlab#285500 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer