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-tw/docs/current/usage/actions/comparison.md

@@ -18,10 +18,7 @@ Gitea Actions 旨在與 GitHub Actions 相容，差異如下。
 <a id="action-prefixes"></a>
 ### Action 前綴
 
-`uses:` 接受以下前綴：
-
-- `self:` 參照您自己的 Gitea 實例，例如 `uses: self:owner/repo@v1` 或 `uses: self:owner/repo/.gitea/workflows/build.yml@v1`
-- `builtin:` 執行 Gitea Runner 內建的 Action，無需下載，也不依賴 Node.js，例如 `uses: builtin:checkout`
+`uses:` 接受 `self:` 前綴，用於參照您自己的 Gitea 實例，例如 `uses: self:owner/repo@v1` 或 `uses: self:owner/repo/.gitea/workflows/build.yml@v1`。
 
 ### 使用Go編寫Actions