Change background handling defaults in imageDifference.py

Image differencing tests with DECam and HSC have identified problems with background subtraction using the default ip_diffim configs (DM-20555, DM-20484).

While investigation of these issues is ongoing, the performance improvements from adjusted configuration parameters are notable: ~15% reduction in false positives in DECam, and even larger in HSC. Accordingly we suggest changing the defaults.

If accepted, this RFC would change to the following default configurations in imageDifference.py:

config.differencer.subtract['al'].kernel.active.spatialBgOrder = 2

config.differencer.doAddCalexpBackground = False

The current defaults are 0 and True.