update the Checkstyle tool to v13.0.0 (requires JDK 21 or higher)

Author: stephengold

.github/workflows/push.yml

@@ -5,14 +5,14 @@ name: CI at GitHub
 on: [push]
 
 jobs:
-  Java17-Linux:
+  Java21-Linux:
     if: contains(toJson(github.event.commits), '[ci skip] ') == false
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/setup-java@v5
         with:
           distribution: 'zulu'
-          java-version: 17
+          java-version: 21
       - uses: actions/checkout@v6
       - uses: gradle/actions/wrapper-validation@v5
       - run: ./gradlew build javadoc --console=plain --stacktrace

README.md

@@ -27,15 +27,15 @@ DacWizard was designed for a desktop environment with:
 ## How to build and run DacWizard from source
 
 1. Install a 64-bit [Java Development Kit (JDK)][adoptium],
-   version 17 or higher,
+   version 21 or higher,
    if you don't already have one.
 2. Point the `JAVA_HOME` environment variable to your JDK installation:
    (In other words, set it to the path of a directory/folder
    containing a "bin" that contains a Java executable.
    That path might look something like
-   "C:\Program Files\Eclipse Adoptium\jdk-17.0.3.7-hotspot"
-   or "/usr/lib/jvm/java-17-openjdk-amd64/" or
-   "/Library/Java/JavaVirtualMachines/zulu-17.jdk/Contents/Home" .)
+   "C:\Program Files\Eclipse Adoptium\jdk-21.0.3.7-hotspot"
+   or "/usr/lib/jvm/java-21-openjdk-amd64/" or
+   "/Library/Java/JavaVirtualMachines/zulu-21.jdk/Contents/Home" .)
   + using Bash or Zsh: `export JAVA_HOME="` *path to installation* `"`
   + using [Fish]: `set -g JAVA_HOME "` *path to installation* `"`
   + using Windows Command Prompt: `set JAVA_HOME="` *path to installation* `"`

gradle.properties

@@ -0,0 +1,4 @@
+## configure Gradle properties for building the DacWizard Project
+
+## Checkstyle requires JDK 21 or higher:
+netbeans.hint.jdkPlatform = JDK_21__System_

gradle/libs.versions.toml

@@ -2,7 +2,7 @@
 
 [versions]
 
-checkstyle = "12.3.1"
+checkstyle = "13.0.0"
 jme = "3.9.0-beta1"
 
 [libraries]