CAMEL-23508: doc-sync 4.18 upgrade guide for camel-elasticsearch-rest-client header rename

[oscerd]

Summary

Doc-sync companion to the camel-4.18.x backport PR #23244 for CAMEL-23508.

Per the project policy in CLAUDE.md:

The camel-4x-upgrade-guide-4_XX.adoc files for ALL versions live on main
and act as the canonical history across all releases. When a PR is backported
to a maintenance branch and adds an upgrade-guide note for that release line,
the corresponding entry must ALSO be added to the matching
camel-4x-upgrade-guide-4_XX.adoc file on main.

This PR adds the camel-elasticsearch-rest-client entry to
docs/user-manual/modules/ROOT/pages/camel-4x-upgrade-guide-4_18.adoc
under the Upgrading from 4.18.1 to 4.18.3 section, documenting the rename
of the ElasticSearchRestClientConstant Exchange header string values to the
CamelElasticsearch* convention.

Related

• Original PR (main): CAMEL-23508: camel-elasticsearch-rest-client - align Exchange header constant names with Camel naming convention #23212
• Backport PR (camel-4.18.x): [backport camel-4.18.x] CAMEL-23508: camel-elasticsearch-rest-client - align Exchange header constant names with Camel naming convention #23244

Test plan

• No code changes; doc-only.

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Claude Code on behalf of Andrea Cosentino

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