Suppose you have this config:

```
gitlab_rails['redis_sentinels'] = [
  {"host"=>"gitlabtest-cluster-redis-01", "port"=>26379},
  {"host"=>"gitlabtest-cluster-redis-02", "port"=>26379},
  {"host"=>"gitlabtest-cluster-redis-03", "port"=>26379}
]
gitlab_rails['redis_sentinels_password'] = "some password"
redis['enable'] = false
redis['master_name'] = "gitlab-redis"
redis['master_password'] = "some password"
```

Previously ActionCable Redis settings would only look for
`gitlab_rails['redis_actioncable_sentinels_password']` and did
not fall back to `gitlab_rails['redis_sentinels_password']`.

Relates to https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/8728

Changelog: fixed