Support Building Against a Different JDK Version

Can use -PtestToolchain=17 to test the compatibility
against JDK 17

buildSrc/src/main/groovy/io/spring/gradle/convention/AbstractSpringJavaPlugin.groovy

@@ -65,8 +65,16 @@ public abstract class AbstractSpringJavaPlugin implements Plugin<Project> {
         project.test {
             useJUnitPlatform()
         }
+		project.java.sourceCompatibility = toolchainVersion(project)
 		additionalPlugins(project);
 	}
 
 	protected abstract void additionalPlugins(Project project);
+
+	private Integer toolchainVersion(Project project) {
+		if (project.hasProperty("testToolchain")) {
+			return project.property("testToolchain").toString().toInteger();
+		}
+		return 17;
+	}
 }

buildSrc/src/main/groovy/io/spring/gradle/convention/SpringModulePlugin.groovy

@@ -40,7 +40,6 @@ class SpringModulePlugin extends AbstractSpringJavaPlugin {
 		if (!Utils.isRelease(project)) {
 			deployArtifacts.dependsOn project.tasks.artifactoryPublish
 		}
-		project.java.sourceCompatibility = JavaVersion.VERSION_17
 	}
 
 }

buildSrc/src/main/groovy/io/spring/gradle/convention/SpringTestPlugin.groovy

@@ -14,9 +14,9 @@
  * the License.
  */
 
-package io.spring.gradle.convention;
+package io.spring.gradle.convention
 
-import org.gradle.api.Project;
+import org.gradle.api.Project
 
 /**
  * @author Rob Winch