Expose default_git_depth via project API

Closes #62908

See merge request gitlab-org/gitlab-ce!29353

Enable Get and Update of ci_default_git_depth for
Project API.

Renaming Project#default_git_depth to :ci_default_git_depth
to give more context through the API usage.

Add API documentation

Adds `set_issue_updated_at` similar to `set_issue_created_at`
permission and cleans up the related permission check in issues
API.

Add spec for task_completion_status

Add test cases for task_completion_status result

Extracted shared samples

Add new spec file for task completion status response

Fix style errors

Add changelog entry

Changed samples to Hashes

Remove test for successful request

Remove not nil expectation

Add task_completion_status to api documentation for issues

Add task_completion_status to api documentation for merge_requests

Refactor spec so it just requests one specific item

Add task_completion_status to Taskable

Simplified task completion status in entities

Refactor spec so it separates status code check and content check

Fix spec description text and field name

Signed-off-by: Rémy Coutable <remy@rymai.me>

Adjustments related to DiffNotes on diffs outside of a project's main repository

See merge request gitlab-org/gitlab-ce!29023

Automatically update MR merge-ref along merge status

Closes #58495

See merge request gitlab-org/gitlab-ce!28513

This turns Notable::RESOLVABLE_TYPES into a
`Notable.resolvable_types`. That allows us to override it in EE.

We have one auto merge strategy today - Merge When Pipeline
Succeeds.

In order to add more strategies for Merge Train feature,
we abstract the architecture to be more extensible.

Removed arguments

Fix spec