This is the continuation of #28363 , this time about getting gossiper scheduling group via database.
Several places that do it already have gossiper at hand and should better get the group from it.
Eventually, this will allow to get rid of database::get_gossip_scheduling_group().
Refining inter-components API, not backporting
Closesscylladb/scylladb#28412
* github.com:scylladb/scylladb:
gossiper: Export its scheduling group for those who need it
migration_manager: Reorder members