update the Gradle build tool to v9.0.0, JDK 17+ required to build

Author: stephengold

.github/workflows/push.yml

@@ -5,25 +5,14 @@ name: CI at GitHub
 on: [push]
 
 jobs:
-  Java8-Linux:
-    if: contains(toJson(github.event.commits), '[ci skip] ') == false
-    runs-on: ubuntu-22.04
-    steps:
-      - uses: actions/setup-java@v4
-        with:
-          distribution: 'zulu'
-          java-version: 8
-      - uses: actions/checkout@v5
-      - run: ./gradlew build install --console=plain --stacktrace
-
-  Java11-Linux:
+  Java17-Linux:
     if: contains(toJson(github.event.commits), '[ci skip] ') == false
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/setup-java@v4
         with:
           distribution: 'zulu'
-          java-version: 11
+          java-version: 17
       - uses: actions/checkout@v5
       - uses: gradle/actions/wrapper-validation@v4
       - run: ./gradlew build install --console=plain --stacktrace

gradle/wrapper/gradle-wrapper.jar

@@ -1,6 +1,6 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.14.3-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.0.0-bin.zip
 networkTimeout=10000
 validateDistributionUrl=true
 zipStoreBase=GRADLE_USER_HOME

gradle/wrapper/gradle-wrapper.properties

@@ -26,24 +26,20 @@ checkstyle {
 
 tasks.withType<JavaCompile>().all { // Java compile-time options:
     options.compilerArgs.add("-Xdiags:verbose")
-    if (javaVersion.isCompatibleWith(JavaVersion.VERSION_14)) {
-        // Suppress warnings that source value 7 is obsolete.
-        options.compilerArgs.add("-Xlint:-options")
-    }
+
+    // Suppress warnings that source value 7 is obsolete:
+    options.compilerArgs.add("-Xlint:-options")
+
     options.compilerArgs.add("-Xlint:unchecked")
     options.setDeprecation(true) // to provide detailed deprecation warnings
     options.encoding = "UTF-8"
-    if (javaVersion.isCompatibleWith(JavaVersion.VERSION_1_10)) {
-        options.release = 8
-    }
+    options.release = 8
 }
 
 tasks.withType<Javadoc>().all {
-    // Disable doclint for JDK8+.
-    if (javaVersion.isJava8Compatible()) {
-        (options as CoreJavadocOptions).apply {
-            addStringOption("Xdoclint:none", "-quiet")
-        }
+    // Disable doclint:
+    (options as CoreJavadocOptions).apply {
+        addStringOption("Xdoclint:none", "-quiet")
     }
 }