Once the Approximate code in afw.math has been cleaned up (see DM-2920), set the default for background subtraction to be the Chebychev Approximation (i.e. useApprox=True). Ensure any other relevant config defaults (e.g. binSize, approxOrderX) are adjusted appropriately. This will change the outputs of the lsst_dm_stack_demo, so the "expected" files will need to be replaced along with this change in default settings (see DM-2778 for some comparisons of the demo outputs using the interpolation vs. approximation background estimation schemes).