Feat: downgrade subepics

related to #1303 (closed)

What does this MR do and why?

• Move subepics to premium

Screenshots or screen recordings

Screenshots for Premium Plan

header	Epic Roadmap	Epic List	Epic Detail	Issue Detail
Before
After

How to set up and validate locally

• Setup the group with a Premium plan
• Login with a group user
• Create an epic
• Goto epic detail
• Click on Add button
• You should see epic dropdown list

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.