Un-deprecate ygot.GoStruct, revert ygot.ValidatedGoStruct to pre-v0.18.1 (#664)

* Attempt to make ValidatedGoStruct backwards-compatible

* Un-deprecate `ygot.GoStruct`, revert `ygot.ValidatedGoStruct`

* fix some comments

* Fix staticcheck

* Improve comment

Author: wenovus

demo/device/devicedemo.go

@@ -111,7 +111,7 @@ func CreateDemoDeviceInstance() (*oc.Device, error) {
 	if err != nil {
 		return nil, err
 	}
-	// Since ygot.MergeStructs returns an ygot.ValidatedGoStruct interface, we
+	// Since ygot.MergeStructs returns an ygot.GoStruct interface, we
 	// must type assert it back to *oc.Device.
 	return mergedDev.(*oc.Device), nil
 }