Public project-level approval rule API

The port to CE is here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31455

What does this MR do?

Add new public project-level approval rule API. The following endpoints have been added:

• GET /projects/:id/approval_rules
• POST /projects/:id/approval_rules
• PUT /projects/:id/approval_rules/:approval_rule_id
• DELETE /projects/:id/approval_rules/:approval_rule_id

The private API isn't changed and still behaves the same way. Deprecate old approval APIs.

Related to #11877 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers