Place "Hide dismissed vulnerabilities" toggle behind a feature flag

• Related issue: #9102 (closed)
• Related MR: !15333 (merged)

What does this MR do?

The MR that added the Hide dismissed vulnerabilities toggle was merged before the backend MR could be completed. The present MR puts the toggle behind the hide_dismissed_vulnerabilities feature flag to prevent it to be released in an incomplete state.

How to test?

• Start a Rails console from the gitlab/ directory:

rails console

• Enable the feature flag:

Feature.enable(:hide_dismissed_vulnerabilities)

• Visit a group or project security dashboard

Screenshots

Feature flag disabled	Feature flag enabled

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Style guides

Performance and Testing

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

Close #31865 (closed)