Indices Now Support Nullable Types with On-Disk Format Changes

Now indices can handle Nullable types, including isNull and isNotNull. #12433 and #12455 (Amos Bird) and #27250 (Azat Khuzhin). But this was done with on-disk format changes, and even though new server can read old data, old server cannot. Also, in case you have MINMAX data skipping indices, you may get Data after mutation/merge is not byte-identical error, since new index will have .idx2 extension while before it was .idx. That said, that you should not delay updating all existing replicas, in this case, otherwise, if old replica (<21.9) will download data from new replica with 21.9+ it will not be able to apply index for downloaded part.