ci: update to actions/checkout@v6

[clado]

CI is showing a lot of warnings (72) right now. They are generally of the following type:

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. 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/

I don't anticipate that bumping the checkout action to v6 / Node.js 24 will cause any issues. (While Node.js 24 drops support for ARM32 and macOS versions <= 13.4, this shouldn't break anything because we use Docker to test in those environments, not github runners natively.) But! If it does cause issues it's probably better to find out now rather than by surprise 3 months from now... :)

[rustbot]

r? @sayantn

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• Owners of files modified in this PR: @Amanieu, @folkertdev, @sayantn
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