CAMEL-23515: docs - sync camel-nats 4.14 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-4.14.x): [backport camel-4.14.x] CAMEL-23515: camel-nats - Use dedicated HeaderFilterStrategy aligned with sibling components #23255

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 4.14.x backport (#23255) adds the camel-nats
header-filter note to camel-4x-upgrade-guide-4_14.adoc on the maintenance
branch; this PR adds the matching entry to camel-4x-upgrade-guide-4_14.adoc
on main so the canonical history does not drift.

Change

Adds a === camel-nats section to camel-4x-upgrade-guide-4_14.adoc
documenting the new default NatsHeaderFilterStrategy.

Docs-only; no code or build impact.

Note: camel-4x-upgrade-guide-4_14.adoc on main is broadly behind the
camel-4.14.x branch copy (several maintenance-branch sections such as
camel-couchdb / camel-couchbase / Default-deserialization are absent on
main). That is separate pre-existing drift, intentionally left out of
scope here to keep this PR focused on the CAMEL-23515 entry only.

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

Claude Code on behalf of Andrea Cosentino

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