chore: sync camel-4x-upgrade-guide-4_18.adoc with camel-4.18.x branch

[davsclaus]

Summary

• Sync the camel-http gzip double-decompression upgrade guide entry (CAMEL-24234) from the camel-4.18.x backport PR (CAMEL-24234: Backport gzip double-decompression fix to camel-4.18.x #25073) to the main branch.

Per the backport upgrade-guide policy, all version-specific upgrade guide entries must also exist on main as the canonical history.

Test plan

• Doc-only change, no tests needed

Claude Code on behalf of Claus Ibsen (@davsclaus)

🤖 Generated with Claude Code

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🧪 CI tested the following changed modules:

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