Please upgrade to nodejs 22

When using the holonix v0.5 flake, I ran into an issue where my M1 running Sequoia 15.5 was attempting to build nodejs 20 from scratch because it has been moved into maintenance mode and there are no longer cached binaries. Additionally, the build was failing for some obscure error. My work around was to create my own custom holonix flake that got rid of both hc-launch and holochain-playground so it would honor my request for the current LTS version of node (see https://github.com/nodejs/release#release-schedule).

Additionally, when reading through the current flake.nix I see it is actually requesting nodejs 18, which is not supported by either nixpkgs or nodejs, so getting nodejs 20 was actually a bit of a fluke in that it was the earliest specified and supported version in all the flake.nix files included.

Start by reviewing the current flake.nix and the included flake.nix files mentioned in the issue, focusing on their requested Node.js versions and references to hc-launch and holochain-playground. Update the configuration to use the current LTS version and verify that the flake no longer requests the unsupported Node.js 18 or triggers the reported build problem.

Written by the indexing model from the issue text.