Replace 'issue-open' icon with 'issues' icon

What does this MR do?

Replace non-descript icon with new icon. Once complete, the old can be deprecated from the SVG library.

Replace with to represent an open issue.

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

N/A

Security

N/A