Update the actions runtime to Node.js 20

[peter-murray]

GitHub Actions is currently generating warnings about updating the action to Node.js 20, which needs to be set in the actions.yml file.

See here for more details: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

[cx-miguel-silva]

I am having the same problem. The action is flagging itself.

[stevehipwell]

It looks like this issue could be closed if a new release was cut?

[spiffcs]

Thanks for the bump @stevehipwell - I'll try and get a new release cut this week

[kzantow]

This was done in https://github.com/anchore/scan-action/pull/278