Update Checkout to v6

[jsoref]

v4 triggers warnings of the form:

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/

[Project516]

v7 is latest

[jsoref]

@sethvargo, your profile claims you work for LinkedIn, and LinkedIn says you left Google in May 2025 (around when I opened this PR)*. Unfortunately, that makes you the last person to work on this repository: fc21748. Do you know anyone from your previous team who could look into this repository? (There's no point in me updating the PR further if the repository is abandoned. But if it's abandoned, then I suppose a ticket + PR to archive the repository would be in order.)

*OTOH, https://api.github.com/users/sethvargo says you're GitHub staff (it also, confusingly lists your company as LinkedIn, even though the sidebar says GitHub).

[sethvargo]

@jsoref sorry, I left Google about a year ago. I'm not sure who would be best to review this. And yes, I work for both LinkedIn and GitHub (and Microsoft) 😄.