CAMEL-23577: doc-sync 4.18.x/4.14.x header-rename upgrade-guide entries to main

[oscerd]

Motivation

Per the project's backport-upgrade-guide policy, the camel-4x-upgrade-guide-4_XX.adoc files on main are the canonical history across all release lines. The 2026-05 Exchange-header-constant rename cascade (umbrella CAMEL-23577) was backported to camel-4.18.x and camel-4.14.x, and each backport added its upgrade-guide entry on the respective maintenance branch — but those entries were never mirrored back into the canonical guides on main. As a result main's 4_18 / 4_14 guides were missing the rename family that actually shipped on the LTS branches.

What this PR does

Copies the missing sections verbatim from the maintenance branches into the matching guide files on main, inserted right after the existing === camel-jira section so they group with the rest of the rename family. Documentation only — no code changes.

camel-4x-upgrade-guide-4_18.adoc — 11 sections added:
pdf, arangodb, jt400, mail, github2, elasticsearch / opensearch, google-functions / google-secret-manager, openstack, shiro, web3j, milo

camel-4x-upgrade-guide-4_14.adoc — 8 sections added:
pdf, arangodb, jt400, mail, openstack, shiro, web3j, google-functions / google-secret-manager

Notes:

• github2 and milo are intentionally 4.18-only — those components do not exist on the 4.14.x branch.
• The elasticsearch / opensearch 4_14 entry was already present on main (restored via CAMEL-23581: restore camel-elasticsearch / camel-opensearch section in 4_14 upgrade guide #23611), so it is not re-added here.

Verification

• No conflict markers; no empty sections (every added heading has its full body).
• Section bodies preserved verbatim from the maintenance branches (e.g. milo 28 lines, es+os 62 lines on 4_18).
• Sections placed under the correct version heading (== Upgrading from 4.18.1 to 4.18.3 on 4_18; top/current section on 4_14), adjacent to the existing camel-jira rename entry.

Reviewers

@davsclaus @gnodet

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