Default enable vuln_mgmt custom role

What does this MR do and why?

This MR default enables the custom role work recently completed for vuln_mgmt. The flag has been previously enabled in production for a period of 24hours https://gitlab.slack.com/archives/C101F3796/p1686725209352489 and the functionality was tested.

Relates to: [Feature flag] Roll out `custom_roles_vulnerabi... (#409387 - closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.