Fix: Update default branch of Mono & Manifest

What does this MR do and why?

What

将 Mono client 仓库 和 Manifest 仓库 两个仓库的默认分支配置在它们自己的 Class 中，以修复 mono sync 时可能出现的错误：

$ mono sync
fatal: Couldn't find remote ref refs/heads/main-jh
error: in `sync`: revision refs/heads/main-jh in manifests not found

Why

• RepoProject 是管理 Mono client 仓库 的类
• ManifestProject 是管理 Manifest 仓库 的类
• MetaProject 是上面二者共同的父类

在之前的 MR 中，我们将 main-jh 配置在了父类 MetaProject 中，这不合理。

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