Change security dashboard's filters bar background

• Related issue: #196103 (closed)

What does this MR do?

This changes the security dashboard's filters bar background color from #f2f2f2 to #fafafa to align with metrics dashboard filters bar.

This change affects all security dashboards:

• Pipeline (/:group/:project/pipelines/:id/security)
• Project (/:group/:project/security/dashboard)
• Group (/groups/:group/-/security/dashboard)
• Instance (/-/security)

Screenshots

Before	After

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