Upgrade to node20

[dscho]

We need to do this "by spring 2024", see https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

[rimrul]

We probably also need to bump the version on git-for-windows/setup-git-for-windows-sdk, as we get deprecation warnings about that action despite main already using node20

[dscho]

We probably also need to bump the version on git-for-windows/setup-git-for-windows-sdk, as we get deprecation warnings about that action despite main already using node20

Good point. I had bumped the node.js version already a long time ago, but forgot to publish a new tag. Now it's done: https://github.com/git-for-windows/setup-git-for-windows-sdk/releases/tag/v1.9.0

[dscho]

I'll take care of updating this Action.