API for setting project's default description template for merge requests

Description

User wants to use the API to set a project's default description template for merge requests.

Proposal

Enhance Create project and Edit project APIs to accept a new parameter for default description template.

Links / references

• https://gitlab.zendesk.com/agent/tickets/80242

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

• Feature assurance
• Documentation
• Added to features.yml