Update dependencies to avoid deprecated versions

[ACyphus]

@mxie When using this Action in my workflow (which is incredibly helpful 🙌) I get the following warnings:

1. The set-output command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

2. Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: mxie/pagerduty-oncall-action@main. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

I think bumping the Node version to 20 and the @actions/core version to the latest is all that is needed.

Perhaps enabling dependabot version updates on this repository would make it easier 😀 https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/about-dependabot-version-updates

[mat-sylvia-mark43]

@mxie Would this be possible? Node 16 usage in GHA looks to be EOL this year.

[smstone]

I updated to node20 and latest actions/core in #4 to address them.