Log intentionally unsent notifications as DEBUG (#16997)

app/notification.go

@@ -348,7 +348,7 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 				)
 			} else {
 				// register that a notification was not sent
-				a.NotificationsLog().Warn("Notification not sent",
+				a.NotificationsLog().Debug("Notification not sent",
 					mlog.String("ackId", ""),
 					mlog.String("type", model.PUSH_TYPE_MESSAGE),
 					mlog.String("userId", id),
@@ -380,7 +380,7 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 					)
 				} else {
 					// register that a notification was not sent
-					a.NotificationsLog().Warn("Notification not sent",
+					a.NotificationsLog().Debug("Notification not sent",
 						mlog.String("ackId", ""),
 						mlog.String("type", model.PUSH_TYPE_MESSAGE),
 						mlog.String("userId", id),