Create epic modal work item rollout check

What does this MR do and why?

Adds a check to prevent displaying the 'create work item' modal for creating new epics if the work_item_epics_rollout feature flag is disabled.

MR acceptance checklist

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

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Disable/Enable the work_item_epics_rollout and namespace_level_work_items feature flags
2. Check the new epic modal is available when both of the flags above are enabled
3. Check the new epic modal is unavailable if either of the flags above are disabled