Update to v19

[GTonehour]

Thanks for your work on RSC!

React 19 is now live. package.json React dependencies are v18, and README.md still refers to RSC as an experimental feature.

As of 2024 December, an actual framework-less demo of RSC implementation seems hard to find. Given that this demo is about one of the major features of React 19, I think it should be updated to use this new version. In particular, I think api.server.js should make use of the new prerender API.

I don't master RSC enough to make the PR myself.

(If this is supposed to remain a pre-React 19 demo, then could you link to a more recent one?)