chore: Upgrade built-in actions to v6

[TSenter]

Back in September, GitHub deprecated Node 20 on its self-hosted runners. Per their blog post:

Beginning on June 2nd, 2026, runners will begin using Node24 by default.

These workflows should be updated to remove the use of the older built-in actions with ones that use Node 24. Both actions/checkout and actions/setup-node could go to just v5 instead of v6, since that's when they were changed to run on Node 24, but I didn't see anything in the list of breaking changes that should affect consumers:

actions/checkout v6 Notes

• Improved credential security: persist-credentials now stores credentials in a separate file under $RUNNER_TEMP instead of directly in .git/config
• No workflow changes required — git fetch, git push, etc. continue to work automatically
• Running authenticated git commands from a Docker container action requires Actions Runner v2.329.0 or later

actions/setup-node v6 Notes

• Caching is now automatically enabled for npm projects when either the devEngines.packageManager field or the top-level packageManager field in package.json is set to npm. For other package managers, such as Yarn and pnpm, caching is disabled by default and must be configured manually using the cache input.

• The always-auth input has been removed, as it is deprecated and will no longer be supported in future npm releases. To ensure your workflows continue to run without warnings or errors, please remove any references to always-auth from your configuration.

I would also like to note that actions/setup-node@v5 did have one breaking change that might need to be addressed:

• Enabled caching by default with package manager detection if no cache input is provided.

For workflows with elevated privileges or access to sensitive information, we recommend disabling automatic caching by setting package-manager-cache: false when caching is not needed for secure operation.