Use `actions/checkout@v6` and `actions/setup-python@v6` to avoid the deprecated warning in capi-docs checking

[Locked-chess-official]

Feature or enhancement

Proposal:

This is one warning in C API Docs / Check if all C APIs are documented in Complete job:

Warning: Node.js 20 actions are deprecated. The following actions are running on Node.js 20 and may not work as expected: actions/checkout@v4, actions/setup-python@v5. Actions will be forced to run with Node.js 24 by default starting June 2nd, 2026. Please check if updated versions of these actions are available that support Node.js 24. To opt into Node.js 24 now, set the FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true environment variable on the runner or in your workflow file. Once Node.js 24 becomes the default, you can temporarily opt out by setting ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

[hugovk]

We already have #144103 to update the actions, but we want to get Dependabot back in action first. See the PR and #137650 for discussion.

And for next time, there's no need to create issues for trivial changes like this.

[hugovk]

Dependabot is now back in action via rulesets.