Update JDK requirements

These have long been outdated

CONTRIBUTING.md

@@ -5,7 +5,7 @@ Thank you for your interest in contributing to WorldEdit! We appreciate your
 effort, but to make sure that the inclusion of your patch is a smooth process, we
 ask that you make note of the following guidelines.
 
-* **Target Java 16 for source and compilation.** Make sure to mark methods with
+* **Target Java 21 for source and compilation.** Make sure to mark methods with
   ` @Override` that override methods of parent classes, or that implement
   methods of interfaces.
 * **Use only spaces for indentation.** Our indents are 4-spaces long, and tabs

README.md

@@ -23,8 +23,8 @@ Edit the Code
 Want to add new features to WorldEdit or fix bugs yourself? You can get the game running, with WorldEdit, from the code here, without any additional outside steps, by doing the following *four* things:
 
 1. Download WorldEdit's source code and put it somewhere. We recommend you use something called Git if you already know how to use it, but [you can also just download a .zip file](https://github.com/EngineHub/WorldEdit/archive/master.zip). (If you plan on contributing the changes, you will need to figure out Git.)
-2. Install any version of Java greater than or equal to 16.
-   * Note that if you do _not_ install JDK 16 exactly, Gradle will download it for you on first run. However, it is still required to have some form of Java installed for Gradle to start at all.
+2. Install any version of Java greater than or equal to 21.
+   * Note that if you do _not_ install JDK 21 exactly, Gradle will download it for you on first run. However, it is still required to have some form of Java installed for Gradle to start at all.
 3. Open terminal / command prompt / bash and navigate to the directory where you put the source code.
 4. Run **one** of these following commands:
    * Mac OS X / Linux: `./gradlew :worldedit-fabric:runClient`