CAMEL-23516: docs - sync camel-xmpp 4.18 upgrade-guide entry to main

[oscerd]

Summary

Doc-sync follow-up for the camel-xmpp HeaderFilterStrategy change.

• Source PR (main, 4.21): CAMEL-23516: camel-xmpp - Use dedicated HeaderFilterStrategy aligned with sibling components #23283 (merged)
• Backport PR (camel-4.18.x): [backport camel-4.18.x] CAMEL-23516: camel-xmpp - Use dedicated HeaderFilterStrategy aligned with sibling components #23296

Per the project's backport upgrade-guide policy, the version-specific
camel-4x-upgrade-guide-4_XX.adoc files live canonically on main across
all releases. The 4.18.x backport (#23296) adds the camel-xmpp
header-filter note to camel-4x-upgrade-guide-4_18.adoc on the maintenance
branch; this PR adds the matching entry to camel-4x-upgrade-guide-4_18.adoc
on main so the canonical history does not drift.

Change

Adds a === camel-xmpp section to camel-4x-upgrade-guide-4_18.adoc
(right after the === camel-nats section synced via #23252) documenting
the new default XmppHeaderFilterStrategy.

Docs-only; no code or build impact.

JIRA: https://issues.apache.org/jira/browse/CAMEL-23516

Claude Code on behalf of Andrea Cosentino

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