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-oxm/src/test/schema/flight.xsd

@@ -0,0 +1,36 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  ~ Copyright 2002-2021 the original author or authors.
+  ~
+  ~ Licensed under the Apache License, Version 2.0 (the "License");
+  ~ you may not use this file except in compliance with the License.
+  ~ You may obtain a copy of the License at
+  ~
+  ~      https://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing, software
+  ~ distributed under the License is distributed on an "AS IS" BASIS,
+  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~ See the License for the specific language governing permissions and
+  ~ limitations under the License.
+  -->
+
+<schema xmlns="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified"
+        targetNamespace="http://samples.springframework.org/flight"
+        xmlns:tns="http://samples.springframework.org/flight">
+    <element name="flights">
+        <complexType>
+            <sequence>
+                <element name="flight" type="tns:flightType"
+                         maxOccurs="unbounded">
+                </element>
+            </sequence>
+        </complexType>
+    </element>
+    <element name="flight" type="tns:flightType"/>
+    <complexType name="flightType">
+        <sequence>
+            <element name="number" type="long"/>
+        </sequence>
+    </complexType>
+</schema>