Bump ci checkout version

[jmackay2]

I noticed in a recent CI run checkout gave the following warning:

Node 20 is being deprecated. This workflow is running with Node 24 by default. If you need to temporarily use Node 20, you can set the ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true environment variable. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

This just bumps up the checkout action to the latest version to fix this warning.

[mvieth]

Thanks!