docs: update Node.js version in README (#34485)

### Reason for this change

The README references Node.js 14; however, Node.js 14 is end-of-life, and CDK is ending support for it (#33623).

### Description of changes

1. Update Node.js version in the README to reference the current minimum LTS version (>= 20x).
2. Update the Node.js download link to the official guidance.

I previously updated the Contributing Guide in #34464. I just noticed the README also points to outdated version, so updating as well.

Author: CDillinger

README.md

@@ -20,7 +20,7 @@ infrastructure definition and share it without worrying about boilerplate logic.
 
 The CDK is available in the following languages:
 
-* JavaScript, TypeScript ([Node.js ≥ 14.15.0](https://nodejs.org/download/release/latest-v14.x/))
+* JavaScript, TypeScript ([Node.js >= 20.x](https://nodejs.org/en/download))
   * We recommend using a version in [Active LTS](https://nodejs.org/en/about/previous-releases)
 * Python ([Python ≥ 3.8](https://www.python.org/downloads/))
 * Java ([Java ≥ 8](https://www.oracle.com/technetwork/java/javase/downloads/index.html) and [Maven ≥ 3.5.4](https://maven.apache.org/download.cgi))