Upgrade actions/checkout to v6

[mbarbin]

As shown in the CI jobs v4 is triggering a deprecation warning. Bump it to latest tag available.

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 will be forced to run with Node.js 24 by default starting June 2nd, 2026. Node.js 20 will be removed from the runner on September 16th, 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/

[mbarbin]

Apparently the CI is not in good shape on the main branch. This change would have to be queued behind some other fixes.

[shym]

Sorry for letting this get stale. This has been applied on main as part of the last release. Thanks!