In ALTER KEYSPACE, when a datacenter name is omitted, its replication
factor is implicitly set to zero with vnodes, while with tablets,
it remains unchanged.
ALTER KEYSPACE should behave the same way for tablets as it does
for vnodes. However, this can be dangerous as we may mistakenly
drop the whole datacenter.
Reject ALTER KEYSPACE if it changes replication factor, but omits
a datacenter that currently contains tablet replicas.
Fixes: https://github.com/scylladb/scylladb/issues/25549.
Closesscylladb/scylladb#25731
76174d1f7a