[BE] Multiple value streams: Delete value stream MVC

Problem to Solve

This issue focuses on allowing users to switch between their saved value streams:

• Broken down from: #219106 (closed)
• Job story:
  • Delete saved views: When managing multiple value streams, I would like to delete a saved view, so that I can no longer see it in my options.

This is the backend portion.

frontend work is being tracked in #221205 (closed)

Intended users

• Parker (Product Manager)
• Delaney (Development Team Lead)

Proposal

• Whenever a custom value stream is displayed, show an option to "Delete [Value Stream name]" at the bottom of the dropdown. Do not show this option for the default value stream.
• When the "delete" option is clicked, a pop-up a confirmation modal should appear: "Are you sure you want to delete "Special" Value Stream?"

Permissions and Security

• Permissions for creating new Value Streams is the same as those for configuring them generally.

Feature Flag

• Make available to gitlab-org group on gitlab.com
• Make more broadly available after we get feedback from internal users and addressed any concerns about scale

What does success look like, and how can we measure that?

• Adoption and qualitative feedback from internal users
• Identify issues with VSA calculations, missing features, configuration options & new feature ideas

What is the type of buyer?

• This feature will be made available in GitLab Ultimate