search v2: pre-allocate dashboardQueryResult slice capacity for performance (#91536)

* search v2: pre-allocate dashboardQueryResult slice for performance
* use limit parameter for starting slice size

pkg/services/searchV2/index.go

@@ -846,7 +846,7 @@ func (l sqlDashboardLoader) loadAllDashboards(ctx context.Context, limit int, or
 				attribute.Int64("lastID", lastID),
 			))
 
-			rows := make([]*dashboardQueryResult, 0)
+			rows := make([]*dashboardQueryResult, 0, limit)
 			err := l.sql.WithDbSession(dashboardQueryCtx, func(sess *db.Session) error {
 				sess.Table("dashboard").
 					Where("org_id = ?", orgID).