Fix Gradle Java Toolchain configuration

This commit fixes various issues with the configuration of the Gradle
Java toolchain in the build.

First, the configuration of build properties is fixed in the CI pipeline
because it wasn't properly checked.
The JMH plugin is also upgraded and we now configure its toolchain
support.
This commit also rewrites the XJC tasks in the spring-oxm module,
leveraging a Gradle plugin that creates actual compile tasks we can
configure.

See gh-25787

spring-messaging/src/test/java/org/springframework/messaging/handler/invocation/InvocableHandlerMethodTests.java

@@ -166,7 +166,10 @@ public class InvocableHandlerMethodTests {
 
 
 	@SuppressWarnings("unused")
-	private static class Handler {
+	static class Handler {
+
+		public Handler() {
+		}
 
 		public String handle(Integer intArg, String stringArg) {
 			return intArg + "-" + stringArg;
@@ -181,7 +184,7 @@ public class InvocableHandlerMethodTests {
 	}
 
 
-	private static class ExceptionRaisingArgumentResolver implements HandlerMethodArgumentResolver {
+	static class ExceptionRaisingArgumentResolver implements HandlerMethodArgumentResolver {
 
 		@Override
 		public boolean supportsParameter(MethodParameter parameter) {

spring-messaging/src/test/java/org/springframework/messaging/handler/invocation/reactive/InvocableHandlerMethodTests.java

@@ -183,6 +183,8 @@ public class InvocableHandlerMethodTests {
 
 		private AtomicReference<String> result = new AtomicReference<>();
 
+		public Handler() {
+		}
 
 		public String getResult() {
 			return this.result.get();