Use DB hierarchy restrictions

What does this MR do and why?

• uses hierarchy restrictions stored in DB to validate parent/child types when creating new parent/child relationship
• maximum_depth restriction is not validated yet - this will be added in a separate MR

Related to #378801 (closed)

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable db restrictions and okrs_mvc flags

   Feature.enable(:hierarchy_db_restrictions)
   Feature.enable(:okrs_mvc)

2. Validate with mutation which creates new key result under objective:

mutation {
  workItemCreate(input: {projectPath: "flightjs/Flight",title: "test", workItemTypeId: "gid://gitlab/WorkItems::Type/92", hierarchyWidget: {parentId: "gid://gitlab/WorkItem/630"}}) {
    errors
  	workItem {
    	id
    	workItemType {
	      id
  	    name
    	}
    	widgets {
      	... on WorkItemWidgetHierarchy {
          parent {
            id
            title
          }
          availableChildrenTypes {
            nodes{
            	id
            	name
            }
          }
      	  }
    	}
    }
  }
}

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.