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 javadoc 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 javadoc install --console=plain --stacktrace

README.md

@@ -106,6 +106,7 @@ Add to the project’s "pom.xml" file:
 ## How to build Garrett from source
 
 1. Install a [Java Development Kit (JDK)][adoptium],
+   version 17 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

common.gradle

@@ -21,7 +21,5 @@ tasks.withType(JavaCompile).configureEach { // Java compile-time options:
     options.compilerArgs << '-Xlint:unchecked'
     options.deprecation = true // to provide detailed deprecation warnings
     options.encoding = 'UTF-8'
-    if (JavaVersion.current().isCompatibleWith(JavaVersion.VERSION_1_10)) {
-        options.release = 8
-    }
+    options.release = 8
 }

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