ui: fix order of prune keep reasons

two things wrong with the old code:
 * the sort function wants -1, 0 and 1 as a return value for a<b, a==b and a>b
   respectively, not a bool (which a < b returns)
 * we have to sort the newest backups first, since the first reason is
   'keep-last'. until now, we sorted the oldest backup first, resulting
   in the older backups getting the 'keep-last' reason

reported by a user in the forum:
https://forum.proxmox.com/threads/prune-ui-and-prune-schedule-simulator-dont-match.94944/

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

www/datastore/Prune.js

@@ -86,7 +86,7 @@ Ext.define('PBS.Datastore.PruneInputPanel', {
 		let counter = {};
 
 		backups.sort(function(a, b) {
-		    return a["backup-time"] < b["backup-time"];
+		    return b["backup-time"] - a["backup-time"];
 		});
 
 		let ruleIndex = -1;