Use self_and_descendant_ids helper

What does this MR do and why?

This MR changes the self_and_descendants.select(:id) to use the self_and_descendant_ids helper to locate subgroup ids. In some cases the new version performs better because we have an optimization for loading subgroup ids from a de-normalized table: &11469

If you want to test both versions (optimized, non-optimized) of the self_and_descendant_ids query, you can do the following:

Group.first.self_and_descendant_ids.to_a
Feature.enable(:group_hierarchy_optimization)
# Should run a different query
Group.first.self_and_descendant_ids.to_a

MR acceptance checklist

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