Colorbars for orphaned mappables are deprecated, but no longer raise

Before 3.6.0, Colorbars for mappables that do not have a parent axes would

steal space from the current Axes. 3.6.0 raised an error on this, but without

a deprecation cycle. For 3.6.1 this is reverted, the current axes is used,

but a deprecation warning is shown instead. In this undetermined case users

and libraries should explicitly specify what axes they want space to be stolen

from: ``fig.colorbar(mappable, ax=plt.gca())``.