Update Code Sandbox CI to Node 20 to Match .nvmrc (#34329)

## Summary
Update the CodeSandbox CI configuration to use Node 20 instead of Node
18, so that it matches the Node version specified in .nvmrc. This
ensures consistency between local development environments and CI
builds, reducing the risk of version-related build issues.

Closes #34328

## How did you test this change?
- Verified that .nvmrc specifies Node 20 and .codesandbox/ci.json is
updated accordingly.
- Locally switched to Node 20 using nvm use 20 and successfully ran
build scripts for all packages: `react`, `react-dom`,
`react-server-dom-webpack`, and `scheduler`.
- Confirmed there are no Node 20–specific build errors or warnings
locally.
- CI on the feature branch will now run with Node 20, and all builds are
expected to succeed.

.codesandbox/ci.json

@@ -1,7 +1,7 @@
 {
   "packages": ["packages/react", "packages/react-dom", "packages/react-server-dom-webpack", "packages/scheduler"],
   "buildCommand": "download-build-in-codesandbox-ci",
-  "node": "18",
+  "node": "20",
   "publishDirectory": {
     "react": "build/oss-experimental/react",
     "react-dom": "build/oss-experimental/react-dom",