Setting default vector_index_type to bbq_hnsw for so_vector

[pmpailis]

Now that bbq_hnsw is our default for vectors with >= 384 dims, we can change the so_vector track to make use of this default as well.

Related ES PR: elastic/elasticsearch#129825

[john-wagster]

lgtm

[esbenchmachine]

@pmpailis
A backport is pending for this PR. Please add all required vX.Y version labels.

• If it is intended for the current Elasticsearch release version, apply the corresponding version label.
• If it also supports past released versions, add those labels too.
• If it only targets a future version, wait until that version label exists and then add it.
  (Each rally-tracks version label is created during the feature freeze of a new Elasticsearch branch).

Backporting entails:

1. Ensure the correct version labels exist in this PR.
2. Ensure backport PRs have backport label and are passing tests.
3. Merge backport PRs (you can approve yourself and enable auto-merge).
4. Remove backport pending label from this PR once all backport PRs are merged.

Thank you!