Update JDK recommendations for contributors (#8818)

CONTRIBUTING.md

@@ -9,7 +9,7 @@ This page provides information about contributing code to the Jenkins core codeb
 1. Fork the repository on GitHub
 2. Clone the forked repository to your machine
 3. Install the necessary development tools. In order to develop Jenkins, you need the following:
- - Java Development Kit (JDK) 11 or 17.
+ - Java Development Kit (JDK) 11, 17 or 21.
  In the Jenkins project we usually use [Eclipse Temurin](https://adoptium.net/) or [OpenJDK](https://openjdk.java.net/), but you can use other JDKs as well.
  - Apache Maven 3.8.1 or above. You can [download Maven here](https://maven.apache.org/download.cgi).
  In the Jenkins project we usually use the most recent Maven release.