Fix build with JUnit 6: set Java 17 on test configurations (#310)

JUnit 6.0.1 requires Java 17+. Rather than bumping the plugin's
sourceCompatibility (which would break users on Java 11-16), directly
override the org.gradle.jvm.version attribute on testCompileClasspath
and testRuntimeClasspath to 17. This allows Gradle variant resolution
to accept JUnit 6 artifacts while keeping the plugin bytecode
compatible with Java 11.

The test task already uses a Java 17 JVM via the javaLauncher toolchain.

Co-authored-by: GitHub Copilot <copilot@github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

build.gradle

@@ -1,3 +1,5 @@
+import org.gradle.api.attributes.java.TargetJvmVersion
+
 plugins {
     id 'eclipse'
     id 'java-gradle-plugin'
@@ -31,7 +33,7 @@ configurations {
     compile.extendsFrom plugin
 }
 
-def jUnitVersion = '5.14.0'
+def jUnitVersion = '6.0.1'
 
 dependencies {
     implementation gradleApi()
@@ -72,7 +74,21 @@ configurations {
     }
 }
 
+configurations.testCompileClasspath {
+    attributes {
+        attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 17)
+    }
+}
+configurations.testRuntimeClasspath {
+    attributes {
+        attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 17)
+    }
+}
+
 test {
+    javaLauncher = javaToolchains.launcherFor {
+        languageVersion = JavaLanguageVersion.of(17)
+    }
     useJUnitPlatform()
 
     testLogging {