Exclude any type in supportedConversionTypes for epics

What does this MR do and why?

As Epics do not support converting to other types from UI, we should prevent any type from getting included in supportedConversionTypes field.

References

• gitlab-org#19879 (closed)

Screenshots or screen recordings

NA

How to set up and validate locally

Run the following query on master and then the MR branch to see how supportedConversionTypes returns empty array with Epic type for current branch compared to master.

query namespaceWorkItemTypes {
  namespace(fullPath: "gitlab-org") {
    id
    name
    workItemTypes(name: EPIC, onlyAvailable: true) {
      nodes {
        id
        name
        supportedConversionTypes {
          id
          name
        }
      }
    }
  }
}

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #582916 (closed)