CAMEL-24677: document the camel-smooks external-entity change in the 4.22 and 4.18 upgrade guides

[oscerd]

Adds the camel-smooks external-entity upgrade note to the version-specific upgrade guides on main, following the backports:

• 4.22.0 to 4.22.1 section of the 4.22 guide — backport merged in [backport camel-4.22.x] CAMEL-24677: camel-smooks - align XML reader configuration with the other XML components #26327.
• 4.18.4 to 4.18.5 section of the 4.18 guide — backport [backport camel-4.18.x] CAMEL-24677: camel-smooks - align XML reader configuration with the other XML components #26329.

Per the project's backport upgrade-guide policy, the version-specific upgrade guides live on main, so these entries are added here rather than on the maintenance branches.

Docs-only.

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