Align state of Vulnerability and Finding

What does this MR do?

This MR aligns the states of the new 1st class vulnerabilities between the frontend and the backend.

See this issue for a bit more context: #196541 (closed)

See these issues for a whole lot of context (probably not necessary): #13561 (closed) and #10242 (closed)

The state that was opened will become detected and the state that was closed will become dismissed.

In the interest of keeping this effort moving and not blocking frontend work, I will also be opening another MR to change the closed_by and closed_at columns in the database to be dismissed_by and dismissed_at.

All functionality related to 1st class vulnerabilities is behind the :first_class_vulnerabilities feature flag that is currently turned off.

Screenshots

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