docs: add changelog entry for MongoDB ${var} syntax for collection

changes/ee/feat-10294.en.md

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+When configuring a MongoDB bridge, you can now use the ${var} syntax to reference fields in the message payload within the collection field. This enables you to select the collection to insert data into dynamically.

changes/ee/feat-10294.zh.md

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+在配置 MongoDB 桥时，现在可以使用 ${var} 语法来引用消息负载中的字段，以便动态选择要插入的集合。