Add secure_vulnerability_training FF and vulnerability management tab

What does this MR do and why?

Create the secure_vulnerability_training feature flag and add the "vulnerability management" tab in the security configuration

This MR is part of the new security training feature, please see the epic for the details > &6176 (closed).

Note: the security training section will be fully-added in a separate MR

How to set up and validate locally

This feature is behind a feature flag, to enable

rails c

Feature.enable(:secure_vulnerability_training)

1. Navigate to Security & Compliance > Configuration
2. You will see the "Vulnerability Management" tab

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #346074 (closed)