Instead of moving bone collections by absolute index, they can now be
moved by manipulating `.child_number`. This is the relative index of the
bone collection within the list of its siblings.
This replaces the much more cumbersome `collections.move_to_parent()`
function. Since that function is now no longer necessary (it's been
replaced by assignment to `.parent` and `.child_number`), it's removed
from RNA. Note that this function was never part of even a beta build of
Blender.
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