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.