fix: use DateTime.UtcNow in ElasticSearch store for consistency

All other stores use UTC timestamps; ElasticSearch was using local time.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: alexeyzimarev

src/Experimental/src/Eventuous.ElasticSearch/Store/ElasticEventStore.cs

@@ -31,7 +31,7 @@ PersistedEvent AsDocument(NewStreamEvent evt, long position)
                 (ulong)position + 1,
                 evt.Payload,
                 evt.Metadata.ToHeaders(),
-                DateTime.Now
+                DateTime.UtcNow
             );
     }