Update to node20 and update eslint

[Wissididom]

Since GitHub is deprecating Node 16 (https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/) I thought I'd do a PR updating it to Node 20. The only files I touched were .nvmrc, CHANGELOG.md, action.yml and package.json, all other files were updated by npm.
While I was at it, I also updated eslint even tho I don't mind reverting that change if necessary.
For .nvmrc I just took the version I found on https://nodejs.org as LTS.
For CHANGELOG.md I copied and adjusted it from further down in an earlier version when the update to Node 16 happened.

I don't know why the diff shows that something was changed in line 263 of CHANGELOG.md

Test PR: Wissididom#1

[dangoslen]

Thanks for this update! I'll cut a release for it soon