Update the GitHub Actions workflows to use the current Node.js LTS version

[Snuffleupagus]

The active LTS version is now based on Node.js version 20, hence let's update the relevant workflows to use that one instead; see https://en.wikipedia.org/wiki/Node.js#Releases

Given that we still support Node.js version 18, i.e. the maintenance LTS version, in the PDF.js library we'll keep testing both versions in GitHub Actions to prevent regressions.

[Snuffleupagus]

I don't understand why the "Test"-task no longer reports success, since both jobs passed, but I'm probably missing something regarding the configuration...

[timvandermeij]

The configuration looks good, but I think GitHub got confused somehow, perhaps because of force-pushes while the actions were still running. I have retriggered them to verify this.

[timvandermeij]

Reading threads like https://github.com/orgs/community/discussions/26698, https://github.com/orgs/community/discussions/30724 and https://stackoverflow.com/a/71983537 it looks like it's because the matrix approach changes the names of the test steps, and the fact that the currently named task is marked as required while the matrix-named tasks are not indicates that we need to change the branch protection rule configuration for this repository. Sadly only administrators can do this, so maybe @calixteman can chime in for this; could you perhaps check the branch protection rules for this repository and update it to the new names? (Hopefully we can do this with some kind of regex like "Test.*" so that we don't have to change this next time a new test name gets introduced if the matrix is changed.)

[timvandermeij]

The change itself is correct, so I'm approving this, but we can only merge it once the branch protection rules have been updated.