Enable bfloat16 and on-disk rescoring for dense vectors

[thecoop]

Remove the feature flag for bfloat16 and on_disk_rescore. bfloat16 is marked as GA for 9.3, on_disk_rescore is in preview for now.

Doc changes: elastic/docs-content#3847
Spec changes: elastic/elasticsearch-specification#5728

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Hi @thecoop, I've created a changelog YAML for you.

[github-actions]

🔍 Preview links for changed docs

• docs/reference/elasticsearch/mapping-reference/dense-vector.md

[github-actions]

ℹ️ Important: Docs version tagging

👋 Thanks for updating the docs! Just a friendly reminder that our docs are now cumulative. This means all 9.x versions are documented on the same page and published off of the main branch, instead of creating separate pages for each minor version.

We use applies_to tags to mark version-specific features and changes.

Expand for a quick overview

When to use applies_to tags:

✅ At the page level to indicate which products/deployments the content applies to (mandatory)
✅ When features change state (e.g. preview, ga) in a specific version
✅ When availability differs across deployments and environments

What NOT to do:

❌ Don't remove or replace information that applies to an older version
❌ Don't add new information that applies to a specific version without an applies_to tag
❌ Don't forget that applies_to tags can be used at the page, section, and inline level

🤔 Need help?

• Check out the cumulative docs guidelines
• Reach out in the #docs Slack channel

[elasticsearchmachine]

Hi @thecoop, I've updated the changelog YAML for you. Note that since this PR is labelled release highlight, you need to update the changelog YAML to fill out the extended information sections.

[tteofili]

LGTM