Add a graphql mutation for toggling cvs

What does this MR do and why?

Add mutation to toggle whether a project has continuous vulnerability scans enabled.

This is one of a series of MRs:

• add attribute to the database and model - !131305 (merged)
• update the presenter to make the attribute available - !131315 (merged)
• add mutation to toggle the attribute - 👈 this MR

Note: the target of this MR is the branch introduced by !131305 (merged) to make the attribute available to the presenter. This MR is blocked until !131305 (merged) merges.

This project adds a single graphql mutation:

mutation ProjectSetContinuousVulnerabilityScans($input: ProjectSetContinuousVulnerabilityScansInput!) {
  projectSetContinuousVulnerabilityScans(input: $input) {
    continuousVulnerabilityScanningEnabled
    errors
  }
}

The view state is managed by the security config presenter added in this MR: !131315 (merged)

Screenshots or screen recordings

n/a

How to set up and validate locally

n/a

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 #424374 (closed)