CI: update the checkout action to v6

[oSoMoN]

This addresses the following deprecation warning, soon to become an error:

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/