Currently, node ops virtual task gathers its children from all nodes contained
in a sum of service::topology::normal_nodes and service::topology::transition_nodes.
The maps may contain nodes that are down but weren't removed yet. So, if a user
requests the status of a node ops virtual task, the task's attempt to retrieve
its children list may fail with seastar::rpc::closed_error.
Filter out the tasks that are down in node_ops::task_manager_module::get_nodes.
Fixes: #20843.
Closesscylladb/scylladb#20856
efc7ad8547