Fix flaky 04758_alter_modify_projection_settings_replicated

The test altered the projection settings on one replica and then read the
definition back from the other after `SYSTEM SYNC REPLICA`. That is a
`ReplicatedMergeTree` guarantee only: there the metadata change is a
replication queue entry, so draining the queue applies it.

On `SharedMergeTree` there is no such queue. The sync refreshes parts and
mutations, and only schedules the background task that applies the table
structure, so the following `SHOW CREATE TABLE` could still report the
pre-alter `index_granularity`. `alter_sync` defaults to 1, meaning the
`ALTER` waits for the current replica only, so nothing waited for the
second replica.

Wait with `alter_sync = 2`, which blocks until every replica reports the
new metadata version, and is correct for both engines.

Closes: https://github.com/ClickHouse/clickhouse-private/issues/71837