Currently, it runs in the gossiper scheduling group, because it's
invoked by the topology coordinator. That scheduling group has the
same amount of shares as user workload. Plan-making can take
significant amount of time during rebalancing, and we don't want that
to impact user workload which happens to run on the same shard.
Reduce impact by running in the maintenance scheduling group.
Fixes#26037Closesscylladb/scylladb#26046