use displaced_mesh in the DGKernels

[xiaojbing]

I use displacements = disp_x disp_y in the [Mesh] block, and I also want to use the use_displaced_mesh = true in other block to update the coordinate in the equations. However, in the [DGKernels] ,if I don't write boundary = , and only write use_displaced_mesh = true in the block, it will be crashed; and If I write both the boundary = and use_displaced_mesh = true ,it can run. but seems that the results does not make sense. Is there any way to solve this contradictory problem?

[GiudGiud]

linked to #20583