Bump `client-output-buffer-limit` for `redis` nodes

From https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6228:

Set the hard limit to 4gb as having the same hard and soft limits defies the purpose of the latter.

Note that chef-client doesn't run gitlab-ctl reconfigure on redis nodes, and we can't run it ourselves as it will restart the service upon changing redis.conf. So updating the file manually and running config set <new config> in a Redis console is our best approach. Experiment in staging first!