Fix for versioning old 2.6 node groups, causing dangling link pointers.

In 2.6 the old method of using bNodeSocket lists in bNodeTree directly
as group sockets was replaced with new group input/output nodes. This
required versioning to create those input/output nodes and then redirect
links to the new node sockets. Because creating nodes relies heavily on
node typeinfo this versioning was done in the `_after_linking` section
of the 2.6 versioning code, running after _all other versioning_
(including for much newer versions!) has already happended.

While typinfo is available at that point, doing such late versioning
causes severe problems when the data structure changes, as is the case
with the recent node panels patch (#111348). The new node group
interface also has versioning code for 4.0, but this runs _before_ the
`_after_linking` code for 2.6! Versioning for node panels expects
sockets in bNodeTree to not have any links pointing at them, but this is
not true for old 2.6 files which have not yet been fully versioned at
that point, because of the late versioning stage. Subsequently 2.6
`_after_linking` code crashes when trying to modify node links with
dangling pointers.

The solution here is to move the old versioning code out of the
`after_linking` stage to restore the expected versioning chain. This
requires creating nodes and node sockets without any typeinfo, but
luckily we only need to create simple known group input/output nodes
which don't have much complicated behavior.

@@ -2173,0 +2266,4 @@
+
+    /* Convert the previously used ntree->inputs/ntree->outputs lists to interface nodes.
+     * Pre 2.56.2 node trees automatically have all unlinked sockets exposed already,
+     * see do_versions_after_linking_250.

@@ -2173,0 +2369,4 @@
+    FOREACH_NODETREE_END;
+  }
+
+  if (!MAIN_VERSION_FILE_ATLEAST(bmain, 280, 60)) {