Add tooltip to "unknown" severity badge icon

What does this MR do?

It adds a GlTooltipDirective to the icon of a "unknown" severity, displaying a description to clarify what the status means.

Tooltip text: The rating "unknown" indicates that the underlying scanner doesn’t contain or provide a severity rating.

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

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

Related to #198161 (closed)