Build instructions for prebuilt apps: amend JDK version

static/build.html

@@ -1035,7 +1035,7 @@ <h3><a href="#prebuilt-apps">Prebuilt apps</a></h3>
                     standard <code>gradle</code> Android plugin build system. You can also build those apps from the source code by following these steps:
                     </p>
                     <ul>
-                        <li>Install Gradle, OpenJDK 17 for developers (which should be called <code>java-17-openjdk-devel</code> on Linux), Android Studio, and npm.</li>
+                        <li>Install Gradle, OpenJDK 21 for developers (which should be called <code>java-21-openjdk-devel</code> on Linux), Android Studio, and npm.</li>
                         <li>In the project you want to build, create a <code>local.properties</code> file and inside it, set the path to your JDK e.g. <code>sdk.dir=/home/jane.doe/Android/Sdk</code></li>
                         <li>In Android Studio's Device Manager, make sure you have a device running (virtual device, or physical device paired using wi-fi)</li>
                         <li>From a console in your project folder, run <code>gradle installDebug</code>. This will install the app from the source code, alongside any prebuilt version of the app which you may already have.</li>