Update Node.js Version in devenv.nix (16.x EOL)

[markshust]

The default devenv.nix configuration in Symfony Flex fails to build because it uses Node.js 16.x, which has reached EOL and been removed from Nixpkgs.

Steps to Reproduce:

1. Install Nix and Devenv
2. Create new Symfony Flex project
3. Run devenv up

Error:

error: nodejs_16 has been removed as it is EOL.

Fix: Update devenv.nix to use a supported Node.js version:

languages.javascript = {
  enable = lib.mkDefault true;
- package = lib.mkDefault pkgs.nodejs-16_x;
+ package = lib.mkDefault pkgs.nodejs_20;
};

Why This Fixes It:

• Node.js 16.x reached EOL on September 11, 2023
• Node.js 20.x is the current LTS version
• Removes reference to non-existent package in Nixpkgs
• This affects all new projects using the devenv template

Testing:

• Verified build succeeds with devenv up
• Confirmed Symfony Flex runs correctly with Node.js 20.x

[markshust]

This appears to rather be a problem with recipe used in Shopware Flex, not with this project. Closing this out as a non-issue.