[DO NOT MERGE] Add semantic_text field type to MMR Result Diversification Retriever

[markjhoy]

Adds the ability for the diversify retriever to allow semantic_text fields as a field type to use for the diversification.

Follow on to : #135880

[elasticsearchmachine]

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

[elasticsearchmachine]

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

[markjhoy]

buildkite test this

[github-actions]

🔍 Preview links for changed docs

• docs/reference/elasticsearch/mapping-reference/semantic-text-search-retrieval.md
• docs/reference/elasticsearch/rest-apis/retrievers/diversify-retriever.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

[leemthompo]

Looks good, just few small docs things for me, I can do a final docs review once got tech review :)

[markjhoy]

closing this in favour of the new working version at #141666