[5.x] Fix ordering search results by date

[duncanmcclean]

This pull request fixes an issue where it wasn't possible to order entry search results by date.

Essentially, when you order search results, it uses the DataCollection::multisort() method to perform the ordering. As part of that, it tries to get the value of the date field using this code below:

cms/src/Data/DataCollection.php

Lines 99 to 101 in 7bf2816

	$value = (method_exists($item, $method))
	? call_user_func([$item, $method])
	: $item->get($sort);

The ->get() here defers to the Result::get() method, which will do ->get('date') on the entry. However, since dates aren't part of an entry's data, it was trying to order results by null.

This PR fixes that by checking if the $key is date and deferring to the entry's date method instead.

Fixes #10892.