Set TxId not Step to the maximum value, KIKIMR-14259

ref:30b1ae0420b1e2dbce4ff87c00cd1343a9a91f3d

ydb/core/tx/datashard/datashard_snapshots.cpp

@@ -284,7 +284,7 @@ bool TSnapshotManager::ChangeMvccState(ui64 step, ui64 txId, TTransactionContext
     // mvcc writes before the switch, but they would have happened at the end
     // of the step.
     if (nextVersion) {
-        nextVersion.Step = Max<ui64>();
+        nextVersion.TxId = Max<ui64>();
     }
 
     if (IsMvccEnabled()) {