Allow build JVM to be with Java 21

xtend converts `.last` to `List.getLast()` if the JVM it is running
under has it, so the generated java files will use `List.getLast()`
even though we want to target older JVM. `List.getLast()` was introduced
in Java 21, which is newer than our supported version of Java11.

This is the workaround recommended in eclipse-xtext/xtext#3535

Author: jonahgraham

gradle/versions.gradle

@@ -13,10 +13,10 @@
 version = '1.0.0-SNAPSHOT'
 
 ext.versions = [
-	'xtend_lib': '2.32.0',
 	// Additional JDK version configuration is in java-compiler-settings
 	'java': '11',
 
+	'xtend_lib': '2.40.0',
 	'guava': '[32.1.2,34)',
 	'gson': '[2.9.1,3.0)',
 

org.eclipse.lsp4j.generator/src/main/java/org/eclipse/lsp4j/generator/JsonRpcDataTransformationContext.xtend

@@ -51,7 +51,7 @@ class JsonRpcDataTransformationContext implements TransformationContext {
 	def TypeReference getRightType(TypeReference typeReference) {
 		val type = typeReference.type
 		if (type === eitherType.type) {
-			return typeReference.actualTypeArguments.last
+			return IterableExtensions.lastOrNull(typeReference.actualTypeArguments)
 		}
 		if (type instanceof InterfaceDeclaration) {
 			return type.extendedInterfaces.map[rightType].filterNull.head