Fix "Convert to task" item showing in epics

What does this MR do and why?

Fix "Convert to task" item showing in epics

The "Convert to task" item should not show in epics as we cannot create tasks for epics yet

Changelog: fixed

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Visit any epic
2. Create checklist items in the description
3. Confirm there is no Convert to task dropdown item for the checklist item

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 #422571 (closed)