CAMEL-23528: document camel-neo4j property name validation in the 4.18 upgrade guide

[oscerd]

Sync the 4.18 upgrade-guide entry to main

The camel-4.18.x backport #23294 adds a camel-neo4j entry to camel-4x-upgrade-guide-4_18.adoc. The canonical camel-4x-upgrade-guide-4_XX.adoc files for all versions live on main, so this PR mirrors the same entry into main's copy, keeping the version-specific upgrade guides on main in sync with what ships on camel-4.18.x.

Docs-only. No code change.

Related: #23258, backport #23294
Jira: https://issues.apache.org/jira/browse/CAMEL-23528

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Claude Code on behalf of Andrea Cosentino

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