Remove hide_dismissed_vulnerabilities feature flag

• Related issue: #9102 (closed)

What does this MR do?

This MR removes the hide_dismissed_vulnerabilities feature flag because the feature has been well tested and can be safely enabled by default.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry

NOTE: Since this MR is removing a feature flag, it should have contained the changelog entry for the feature, but since a changelog entry was already added in !15333 (diffs), we can omit it here

• Documentation (if required)
  • Follow up MR: !21141 (merged)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content