Update from actions/checkout@v4 to v6

The v4 version of actions/checkout looks to be using Node.js 20, which is being deprecated. Actions will be forced to run with Node.js 24 by default starting June 2nd, 2026. Here is the full message that appears when a job finishes:

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 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/

Search the repository's GitHub Actions workflow files for references to actions/checkout@v4 and review the surrounding workflow configuration. Update the checkout action to v6, then verify that all relevant references use the requested version and that the workflows remain valid.