zcash_client_sqlite: Mark existing blocks scanned in `shardtree` migration

The `shardtree` migration is applied to a database state that was
created via linear scanning, so we have complete wallet information for
those blocks.

zcash_client_sqlite/src/wallet/init/migrations/shardtree_support.rs

@@ -265,7 +265,7 @@ impl RusqliteMigration for Migration {
                 transaction,
                 Some(ScanRange::from_parts(
                     start..chain_end,
-                    ScanPriority::Historic,
+                    ScanPriority::Scanned,
                 ))
                 .iter(),
             )?;