Update Vulnerability Feedback Documentation

Per #376743 (closed), and with the impending deprecation of vulnerability feedback, our documentation needs to be updated to remove reference to this defunct model/concept.

Implementation Plan

• Remove the feedback entry from https://docs.gitlab.com/ee/user/application_security/terminology/#feedback
• Elaborate what dismissal means in the context of a vulnerability finding: https://docs.gitlab.com/ee/user/application_security/terminology/#finding