Remove obsolete vulnerability_findings_api feature flag

What does this MR do?

This MR renames obsolete mentions of the vulnerability_findings_api feature flag that has become more generic first_class_vulnerabilites in the process of working on the backstage implementation for Standalone Vulnerability objects. Somehow these wrong mentions of the vulnerability_findings_api have survived the MRs merged where this should have been already renamed.

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team