Upgrade actions/checkout to resolve compatibility issue with checkout@v6

[Maoyao233]

Description

This PR upgrades the internal usage of actions/checkout (from v3) to resolve a critical compatibility issue when users use actions/checkout@v6 in their parent workflow.

The Issue

If a user utilizes actions/checkout@v6 in their workflow, they may encounter the following error when this action runs:

remote: Duplicate header: "Authorization"
Error: fatal: unable to access '...': The requested URL returned error: 400

This is caused by a conflict in how Git credentials are persisted between different versions of actions/checkout:

• User Side (v6): As per actions/checkout#2286, newer versions of checkout persist credentials in a separate config file referencing includeIf, instead of modifying .git/config directly.
• Action Side (v3): The current version in this action modifies the local .git/config.
• Conflict: When git fetch/push is executed inside this action, both the global credential (from v6) and the local credential (from v3) are applied, resulting in two Authorization headers and a 400 Bad Request error from GitHub.

Fix

Upgrading the internal action dependency aligns the credential management mechanism and prevents the "Double Auth" conflict. I have verified this fix in a forked version.

[fritx]

Thanks for you feedbacck! ❤️ Maybe we will provide both v3 and v6.

However, there is something wrong with the workflow, will take some time.