optimize: use context.WithCancelCause and context.AfterFunc to simplify streaming lifecycle control

[DMwangnima]

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optimize: 使用 context.WithCancelCause 和 context.AfterFunc 来简化流式的生命周期管理

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1. Kitex 最低要求 go 版本升级至 1.21
2. cancel 功能优化：使用 context.AfterFunc 替代 5s 一次的周期性轮训，cancel 感知和触发更快
3. 使用 context.WithCancelCause 替代原有的 contextWithCancelCause。
   考虑到 streaming 提供的 ctx.Err() 一直会返回协议内部的定制化错误，为了不造成 Breaking Change，contextWithCancelCause 依然保留，只是在 Err() 接口被调用时自动执行 context.Cause 获取真正的错误

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