WIP: Rename vulnerability_findings_api flag to first_class_vulnerabilities

What does this MR do?

A housekeeping MR for First-class vulnerabilities backend MVC implementation to change too narrow-scoped feature flag name vulnerability_findings_api to first_class_vulnerabilities. The reason is that this FF will hide much more changes than just API re-routing (for instance, new APIs for Vulnerabilities and Issues).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance 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