Improve migration path

[GiedriusS]

Converter should update the original meta.json with a hint. StoreGW then should not load those blocks.

[verejoel]

Or maybe a new "migrated to parquet" mark? So the meta.json data structure doesn't need to be modified.

[GiedriusS]

https://github.com/thanos-io/thanos/blob/main/pkg/block/metadata/meta.go#L100 we can use this one, no need to introduce anything new.