DEV: Fix redis.sadd warnings (#23244)

```
Redis#sadd will always return an Integer in Redis 5.0.0. Use Redis#sadd? instead
```

spec/lib/backup_restore/system_interface_spec.rb

@@ -132,7 +132,7 @@ RSpec.describe BackupRestore::SystemInterface do
           key = "#{hostname}:#{pid}"
           process = { pid: pid, hostname: hostname }
 
-          conn.sadd("processes", key)
+          conn.sadd?("processes", key)
           conn.hmset(key, "info", Sidekiq.dump_json(process))
 
           data =