Blender Git Loki

Geometry Nodes: Use multi-input socket in join geometry node

This updates the join node to use the new multi-input socket from D10067.
The change just requires slightly changing the arguments of the exec
function, and changing the socket input list.

Note that this commit does not contain the UI changes, it only allows for
more input links, and combines the two sockets into one. The UI changes
will come next.

Reviewed By: Hans Goudey

Differential Revision: https://developer.blender.org/D10069