Add "closed" as new MR list status

What does this MR do and why?

Related issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/3541

为 Mono 场景下的 MR list 增加 closed 状态。

在之前 MR list 的状态有四种：can_be_merge cannot_be_merge merging 和 merged，没有 closed。如果所有 MR 都关闭了，MR list 状态是 merged，这会让前端 UI 的显示不合理。现在增加一个 closed 状态来体现这种特殊情况。

Screenshots or screen recordings

No UI changes.

A front-end MR will be responsible for changes in the UI.

How to set up and validate locally

参考 !1921 - How to set up and validate locally 的步骤创建 Mono MR 并关闭，然后请求当前 GET 接口。

Before	After

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.