Remove method alias `parent`
On some models we use parent
convenient method to get either project or group where the resource belongs to. The problem is that parent
name is not ideal because it clashes with parent
attribute used for resources which use parent-child hierarchy (especially Epics).
For this reason https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15649 added resource_parent
name but kept method aliases for parent
method. Instead of keeping this alias, we should rename parent
calls to resource_parent
calls.