Fix Arel deprecation warning in clusters_hierarchy
Fix Arel deprecation warning in clusters_hierarchy

As mentioned in https://github.com/rails/rails/pull/29619, this removes
this warning message:

```
Delegating join_sources to arel is deprecated and will be removed in
Rails 6.0. (called from project_clusters_base_query at
app/models/clusters/clusters_hierarchy.rb:62)
```

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/61451

mmkassem/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq