The system_keyspace.hh now includes raft stuff, topology changes stuff, task_manager stuff, etc. It's going to include tablets.hh (but maybe not). Anything that deals with system keyspace, and includes system_keyspace.hh, would transitively pull these too. This header is becoming a central hub for all the features.
This PR removes all the headers from system_keyspace.hh that correspond to other "subsystems" keeping only generic mutations/querying and seastar ones.
Closes#13450
* github.com:scylladb/scylladb:
system_keyspace.hh: Remove unneeded headers
system_keyspace: Move topology_mutation_builder to storage_service
system_keyspace: Move group0_upgrade_state conversions to group0 code
c65bd01174