CAMEL-23526: docs - flag camel-cxf 4.18/4.14 upgrade-guide entries as potential breaking change

[oscerd]

Flags the camel-cxf entries in both camel-4x-upgrade-guide-4_18.adoc and
camel-4x-upgrade-guide-4_14.adoc on main as a potential breaking change,
matching the existing camel-grok - potential breaking change convention and
keeping main's canonical multi-version upgrade-guide history in sync with the
matching changes on the camel-4.18.x and camel-4.14.x maintenance branches.

The CXF Exchange-header constant rename (CAMEL-23526) changes header values
and alters cross-transport propagation — a behaviour change worth flagging in
the section title.

Docs-only; no code change. Per CLAUDE.md's backport upgrade-guide policy.

JIRA: https://issues.apache.org/jira/browse/CAMEL-23526

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PR prepared by Claude Code on behalf of Andrea Cosentino.

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