Require a newer Node version to make Copilot work

Closes #27908

Release Notes:

- Breaking Change: If using system node Zed now requires Node >= v20.
Previously Node >= v18 was required. (Node v18 EOL date is 2025-04-30;
Node v19 EOL since 2023-06-01). Note: This does not change the Zed
bundled Node runtime version (still v23).