Move GridReordering::reorder_cells().

[drwells]

Fixes #12058. This was easier than I thought it would be - the second GridReordering function already used the new-style numbering internally.

[bangerth]

I assume that the code is simply copied without actual changes?

[drwells]

Yup!

[bangerth]

Hm, the testers fail:

/home/runner/work/dealii/dealii/include/deal.II/grid/grid_reordering.h:37:32: error: attribute ignored in declaration of ‘class dealii::GridReordering<dim, spacedim>’ [-Werror=attributes]
 DEAL_II_DEPRECATED_EARLY class GridReordering
                                ^~~~~~~~~~~~~~
/home/runner/work/dealii/dealii/include/deal.II/grid/grid_reordering.h:37:32: note: attribute for ‘class dealii::GridReordering<dim, spacedim>’ must follow the ‘class’ keyword

So it's a placement thing for the attribute.

[drwells]

I can never remember where these things go - it should be fixed now.