FE implementation - Convert Rest API to GraphQL mutation for MR Creation flow

In #220573 (closed), the frontend POSTs to a REST endpoint to trigger the creation of an MR.

This issue tracks the work to migrate over to a GraphQL mutation (backend work tracked by #227465 (closed)).

This is to align with GitLab's value on GraphQL everywhere.

The security_sast_configuration feature flag was added in !40637 (merged), which changed the API of the configureSast mutation. Since this work relies on that mutation to work, we need to do some feature flag juggling to ensure multi-version compatibility across deploys.

Implementation plan

• Enable the security_sast_configuration in production Done via chatops
• Enable the security_sast_configuration in staging Done via chatops
• Merge migration of frontend from REST to GQL mutation: !41995 (merged)
• Verify deployment in production
• Delete the security_sast_configuration in production
• Delete the security_sast_configuration in staging