Remove the builtin:checkout docs (#643)

Follow-up to https://gitea.com/gitea/runner/pulls/1407, which removes the runner's builtin actions. Drops `builtin:checkout` from the develop runner docs and the current comparison page, and lists the removal under the 6.0 breaking changes. Versioned docs keep it, as released runners still ship it, with their `builtin-actions.md` links pinned to the latest 4.x and 5.x tags since the file leaves runner main.

Co-authored-by: Lunny Xiao <[email protected]>
Reviewed-on: https://gitea.com/gitea/docs/pulls/643
Reviewed-by: Lunny Xiao <[email protected]>
Co-authored-by: silverwind <[email protected]>

i18n/zh-cn/docs/current/usage/actions/comparison.md

@@ -18,10 +18,7 @@ Gitea Actions旨在与GitHub Actions兼容，差异如下。
 <a id="action-prefixes"></a>
 ### Action前缀
 
-`uses:`支持以下前缀：
-
-- `self:`引用您自己的Gitea实例，例如`uses: self:owner/repo@v1`或`uses: self:owner/repo/.gitea/workflows/build.yml@v1`
-- `builtin:`运行Gitea Runner内置的Action，无需下载，也不依赖Node.js，例如`uses: builtin:checkout`
+`uses:`支持`self:`前缀，用于引用您自己的Gitea实例，例如`uses: self:owner/repo@v1`或`uses: self:owner/repo/.gitea/workflows/build.yml@v1`。
 
 ### 使用Go编写Actions