API for default description template for issues and merge requests

In scope

• API for the default description template of issues and merge requests, as explained in https://docs.gitlab.com/ee/user/project/description_templates.html#setting-a-default-template-for-issues-and-merge-requests.

Out of scope

• API for the git-source-controled description templates are not in scope, and is covered in https://gitlab.com/gitlab-org/gitlab-ce/issues/44851.

Orginal description

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/34080

We do not have an API for managing many project settings - including merge request templates and approvers. Customer would like an API for these two things, at least. Could this be added to the 'Update project' endpoint as an arg?