Correct number of points of vtkTriQuadraticHexahedron in xdmf

Lukas Schöllerrequested to merge
lukas.schoeller/vtk:fix_xmdf_Hex27 into master

Correct number of vtkTriQuadraticHexahedron for the XDMF3 and XDMF2 reader. Should be 27, instead of 24.

Closes #19088 (closed)

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