CAMEL-23515: docs - sync camel-nats 4.18 upgrade-guide entry to main

[oscerd]

Summary

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

• Source PR (main, 4.21): CAMEL-23515: camel-nats - Use dedicated HeaderFilterStrategy aligned with sibling components #23233 (merged)
• Backport PR (camel-4.18.x): [backport camel-4.18.x] CAMEL-23515: camel-nats - Use dedicated HeaderFilterStrategy aligned with sibling components #23250

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 (#23250) adds the camel-nats 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-nats section to camel-4x-upgrade-guide-4_18.adoc
documenting the new default NatsHeaderFilterStrategy.

Docs-only; no code or build impact.

Note: camel-4x-upgrade-guide-4_18.adoc on main currently has no
=== camel-nats section at all, so the pre-existing 4.18.x JetStream
pull-fix note is also missing from main's copy (separate pre-existing
drift, intentionally left out of scope here to keep this PR focused).

JIRA: https://issues.apache.org/jira/browse/CAMEL-23515

Claude Code on behalf of Andrea Cosentino

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