[4.x] Remove requirement of orderable collection from next/prev tags

[jasonvarga]

At the moment, the collection:next/prev tags only work on dated or orderable collections. "Orderable" collections are ones with a structure set to max depth of 1.

This was because the "order" was ambiguous for multi-depth collections. A while ago in #4883 we made the decision for it to just assume the order is depth-first (how it looks if you read down the page while looking at the tree).

We never looped back and added support for that within the collection:next/prev tags.

This PR removes that requirement. If you want to sort by order, go for it. You can technically even sort a dated collection by order if you add an order field to each entry.

The default sort order of a multi-depth structured collection is title. Since we don't want to make it a breaking change, to get these tags to work, you'll need to add sort="order". In Statamic 5 we will change the default sort order of a multi-depth structured collection to order.