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

* Cleaned issues and issues_statistics docs
* Renamed param with_labels_data to with_labels_details
* Added spec for N+1 check when retrieving labels from issue
* Refactoed CheckAssigneesCount validation class

Remove label_name and milestone_title params support
Add mutually_exclusive validation for author_id and author_username
Add mutually_exclusive validation for assignee_id and assignee_username
Add validation to allow single value for asignee_username on CE
Add separate issue_stats_params helper for statistics params and
reuse in issues_params.

Ability to filter confidential issues

Closes #50747

See merge request gitlab-org/gitlab-ce!24960

This reverts commit 7981c0292b07a0138b096fa082341fcb13e9ce2b, reversing
changes made to 9202bbd129537a698b986e6295d0c783b5a84815.

This reverts commit d133bf84c668df3dfc9938bb04150754cb873c8b, reversing
changes made to 7981c0292b07a0138b096fa082341fcb13e9ce2b.

Ability to filter confidential issues

Closes #50747

See merge request gitlab-org/gitlab-ce!24960

API endpoint for bulk updating issues and MRs

Closes #49449

See merge request gitlab-org/gitlab-ce!25201