gnu: gmsh: Update to 4.6.0.

Gmsh bundles METIS 5 released with Apache License 2.0 since upstream commit
eaff7f6477fe876991615cfa4b5832bd5f12994b.  Instead, Gmsh is linked with the
package 'metis'.

* gnu/packages/maths.scm (gmsh): Update to 4.6.0.
(propagated-inputs): Replace 'opencascade-oce' by 'opencascade-occt', add
'metis'.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>