PHPLIB-1115: Fix waitForSnapshot() for MongoDB 7.0+

Using find instead of aggregate should ensure documents are available in the calling test case on 7.0+ servers.

Author: jmikola

tests/DocumentationExamplesTest.php

@@ -1981,10 +1981,7 @@ private function waitForSnapshot(string $databaseName, string $collectionName):
 
         do {
             try {
-                $collection->aggregate(
-                    [['$match' => ['_id' => ['$exists' => true]]]],
-                    ['session' => $session]
-                );
+                $collection->findOne([], ['session' => $session]);
 
                 break;
             } catch (CommandException $e) {