Remove the Node.js 21 pin in the GitHub Actions CI workflow

[timvandermeij]

In PR #18009 the GitHub Actions CI workflow has temporary been updated to not use the latest Node.js version (22 as of writing) but Node.js 21 instead because the canvas dependency doesn't provide pre-built binaries for Node 22.js yet. We don't want to build it ourselves because that'd require additional complexity for local and workflow installations, and sadly it looks like this may take some time to resolve upstream given that Automattic/node-canvas#2377 and the corresponding release tracking issue Automattic/node-canvas#2232 didn't have recent updates. To avoid having failing builds in the meantime, we chose to pin Node.js to version 21 for the time being.

If Automattic/node-canvas#2377 is resolved upstream and a new release is published with pre-built binaries, we should update to that new version and revert e561a4a so we test against the latest version again.