Geometry Nodes: Only use realized geometry in mesh boolean node

Since 44e4f077 and related commits, geometry nodes doesn't
try to hide the difference between real geometry data and instances from
the user. Other nodes were updated to only support real geometry, but
the "Mesh Boolean" node was never updated and still implicitly gathered
all the instances. This commit removes the special instance behavior in the
boolean node and adds realize instances nodes to keep existing behavior
in most cases. Typically this doesn't make a difference in the result,
though it could in the union mode for instance inputs. Shifting more of
the work to realizing instances should generally be better for
performance, since it's much faster.