Switch to Java 17

Uses the new Java toolchain feature of Gradle to acquire Java 17.

Doing it this way means that you can start ./gradlew under any version
of Java and still use Java 17 for the build; it picks a candidate from
the environment or downloads one if required. It also takes care of
source and target compatibility, with the exception of ide.gradle
which is handled separately.

build.gradle

@@ -134,8 +134,11 @@ subprojects { subproject ->
 
 	apply from: "${rootProject.projectDir}/publish-maven.gradle"
 
-	sourceCompatibility=1.8
-	targetCompatibility=1.8
+	java {
+		toolchain {
+			languageVersion = JavaLanguageVersion.of(17)
+		}
+	}
 
 	[compileJava, compileTestJava]*.options*.compilerArgs = ["-Xlint:none"]
 

ide.gradle

@@ -4,8 +4,8 @@ import org.gradle.plugins.ide.eclipse.model.SourceFolder
 apply plugin: "eclipse"
 
 eclipse.jdt {
-	sourceCompatibility = 1.8
-	targetCompatibility = 1.8
+	sourceCompatibility = 17
+	targetCompatibility = 17
 }
 
 // Replace classpath entries with project dependencies (GRADLE-1116)