Add convenience API key param to remote reindex

[PeteGillinElastic]

This adds a source.remote.api_key parameter to the reindex API. This is equivalent to using source.remote.headers with Authorization set to ApiKey <api_key>. It is a convenience for the user.

Note on docs changes: The example request using an API key now uses an applies-switch to show both the pre-9.3 and post-9.3 / serverless versions. There are a few drive-by improvements to the docs, including:

• Adding some subsections, as suggested by a tech writer.

• Fixing the bug which meant that <OTHER_HOST_URL> was previously not being rendered, because < is a control character.

• Repeating the note about using HTTPS for basic auth in the API key section, as we wouldn't recommend sending API keys over plain HTTP either.

• A nit, but writing <OTHER_HOST_URL>:9200 is strange, since we have a placeholder URL, and the port is part of the URL, so that's fixed.

[elasticsearchmachine]

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

[github-actions]

🔍 Preview links for changed docs

• docs/reference/elasticsearch/rest-apis/reindex-indices.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