Framework: Update Node.js version to v24 LTS

What problem does this address?

The project is currently configured to expect Node.js v20 (minimum version v20.10.0).

This produces a few challenges:

• It's inconsistent with our documentation, which frequently references that we aim to target the current active LTS (example)
• It prevents the use of new features and bug fixes in more recent versions of Node.js
  • Which in turn can introduce the potential for bugs due to lack of awareness of said feature differences (example)
• Version drift increases future overhead when eventually reconciling the drift, and puts us at risk of falling outside the window of support (Node.js v20 will be end-of-life in April 2026)

What is your proposed solution?

Update the expected version of Node.js to v24.x, the current Active LTS.

This may require a few steps, as outlined by @desrosj in https://github.com/WordPress/gutenberg/pull/70844#issuecomment-3160135370 :

• Testing compatibility in both Gutenberg and WordPress Core
• Confirming compatibility with the 3 default themes that have a package.json file and build process
• Coordinating the update to happen in both locations around the same time
• Requesting that the new version be installed on the wordpress.org build server.

An additional step I'd recommend is changing the .nvmrc to specifically pin to the version of Node.js used on WordPress build servers, to ensure everyone is using the same version and to ensure compatibility across build environments.

Some of the work to support thi