Untyped TreeView classes are obsolete

[normanr]

In Unity 6000.5, there are lots of errors like these:

error CS0619: 'TreeView' is obsolete: 'TreeView is now deprecated. You can likely now use TreeView<int> instead and not think more about it. But if you were using that identifier to store InstanceID data, you should instead opt to upgrade your TreeViews to use TreeView<InstanceID> to get the proper typing.'
error CS0619: 'TreeViewState' is obsolete: 'TreeViewState is now deprecated. You can likely now use TreeViewState<int> instead and not think more about it. But if you were using that identifier to store InstanceID data, you should instead opt to upgrade your TreeViews to use TreeViewState<InstanceID> to get the proper typing.'
error CS0619: 'TreeViewItem' is obsolete: 'TreeViewItem is now deprecated. You can likely now use TreeViewItem<int> instead and not think more about it. But if you were using that identifier to store InstanceID data, you should instead opt to upgrade your TreeViews to use TreeViewItem<InstanceID> to get the proper typing.'