Add premium/ultimate missing Project attributes

[ArielPrevu3D]

Description

Add the missing attributes of the Project schema that are marked as "Premium" or "Ultimate" in the Gitlab docs (e.g merge_requests_template).

https://docs.gitlab.com/ee/api/projects.html#templates-for-issues-and-merge-requests

Other "Premium" attributes such as mirror_trigger_builds are missing from the schema typing, event though they are present in the ProjectSchema object.

Proposal

Add merge_requests_template and issues_template to ProjectSchema typing.

Checklist

• [x] I have checked that this is not a duplicate issue.
• [x] I have read the documentation.