CAMEL-23522: doc-sync 4.18 upgrade guide for camel-mail mail.smtp.* gating

[oscerd]

Summary

Doc-sync of the camel-mail mail.smtp.* / mail.smtps.* gating entry from camel-4.18.x onto main's copy of camel-4x-upgrade-guide-4_18.adoc, per the project's backport upgrade-guide policy in CLAUDE.md:

The camel-4x-upgrade-guide-4_XX.adoc files for ALL versions live on main and act as the canonical history across all releases. When a PR is backported to a maintenance branch and adds an upgrade-guide note for that release line, the corresponding entry must ALSO be added to the matching camel-4x-upgrade-guide-4_XX.adoc file on main.

This PR mirrors the entry added in #23381 (backport to camel-4.18.x) onto main, immediately after the existing camel-neo4j entry in the "Upgrading from 4.18.1 to 4.18.3" section.

Linked

• Main PR: CAMEL-23522: camel-mail - gate JavaMail session properties from headers behind opt-in #23362 — CAMEL-23522: camel-mail - gate JavaMail session properties from headers behind opt-in (merged)
• Backport PR: [backport camel-4.18.x] CAMEL-23522: camel-mail - gate JavaMail session properties from headers behind opt-in #23381 — [backport camel-4.18.x] (open)
• Jira: https://issues.apache.org/jira/browse/CAMEL-23522
• Family precedent: CAMEL-23222 / CVE-2025-27636

Test plan

• Single docs file added; no code changes.
• Identical wording to the same section on the camel-4.18.x branch.

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Claude Code on behalf of Andrea Cosentino

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