[4.x] Update entry parent index on collection tree save

[jacksleight]

We've noticed an issue where parent values are not updated in the stache index in certain circumstances, producing incorrect query results. For example:

Create a collection thats's orderable but can have multiple roots, then query all the root entries (no parent):

{{ collection:index_test parent:exists="false" }}
    {{ title }} - <a href="{{ url }}">{{ url }}</a><br>
{{ /collection:index_test }}

That will list all the root entries. Now do either of these:

1. In the tree view move a root entry under a parent, save.
   • Re-run the query above and the entry will still be listed, but it shouldn't.
2. Create a new entry, set the parent, save.
   • Re-run the query above and the new entry will be listed, but it shouldn't.
   • Going into the entry and re-saving corrects this.

There's also https://github.com/statamic/cms/issues/5457 and https://github.com/statamic/cms/issues/5938 which may be the same issue or related to this issue, I'm not 100% sure.

This PR fixes it by updating the existing UpdateStructuredEntryOrder listener to also update the parent index.

I couldn't figure out the right way or place to do the test, if you can point me in the right direction I'll add that in.