[Feature] Vulnerability update notifications

[4naesthetic]

Description

Adds support for sending notifications when the severity of an existing vulnerability associated with a project is updated.

Addressed Issue

https://github.com/DependencyTrack/dependency-track/issues/2361

Additional Details

API Server PR: https://github.com/DependencyTrack/dependency-track/pull/4067

Checklist

• [x] I have read and understand the contributing guidelines
• [x] This PR introduces new or alters existing behavior, and I have updated the documentation accordingly