Rename Vulnerabilities to Findings in Projects security routes

What does this MR do?

This MR adds to the step Rename existing Vulnerabilities API to Vulnerability Findings API of First-class vulnerabilities backend MVC implementation for the Projects routes that power the Project Security Dashboard.

Both old and new routes will be present in the routes table since it's generated upon app initialization and gets preloaded by the webserver. Thus, routing is difficult to change in runtime when the feature flag is switched. That's why we manipulate the routing at the controller level. It's similar to how it's implemented for the Vulnerability Findings REST API.

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