chore: add change logs for v4.4.18

changes/v4.4.18-en.md

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+# v4.4.18
+
+## Enhancements
+
+- Improved the placeholder syntax of the rule engine [#10470](https://github.com/emqx/emqx/pull/10470).
+
+  The parameters of certain actions support using placeholder syntax to dynamically fill in the content of strings. The format of the placeholder syntax is `${key}`.
+  Prior to the improvement, the `key` in `${key}` could only contain letters, numbers, and underscores. Now the `key` supports any UTF8 character after the improvement.
+
+## Bug fixes

changes/v4.4.18-zh.md

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+# v4.4.18
+
+## 增强
+
+- 改进规则引擎的占位符语法 [#10470](https://github.com/emqx/emqx/pull/10470)。
+
+  某些动作的参数支持使用占位符语法，来动态的填充字符串的内容，占位符语法的格式为 `${key}`。
+  改进前，`${key}` 中的 `key` 只能包含字母、数字和下划线。改进后 `key` 支持任意的 UTF8 字符了。
+
+## 修复
+