Unexpected "JRE container" entry derived from pom.xml for Java SE 13

When opening a Maven project which uses maven.compiler.source/target 13, the derived .classpath file uses J2SE-1.5 as its JRE container, instead of the expected JavaSE-12.

In other words: if the value for the Maven compiler properties is unsupported/unrecognized, it should default to the highest currently-supported version. This is much more likely to be what the user wants/expects. For example, with the new JDK release cadence (the first JDK 14 build is already released), it's more common (for me at least) to try out early-access JDK builds.

Environment

• Operating System: Windows 10 Pro 1809 (17763.557)
• JDK version: 12.0.1
• Visual Studio Code version: 1.35.1
• Java extension version: 0.46.0

Steps To Reproduce

1. unzip and open sample-maven-project.zip
2. let the extension generate the project files

Current Result

The .classpath file contains the following:

	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/J2SE-1.5">
		<attributes>
			<attribute name="maven.pomderived" value="true"/>
		</attributes>
	</classpathentry>

Expected Result

The .classpath file contains the following:

	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-12">
		<attributes>
			<attribute name="maven.pomderived" value="true"/>
		</attributes>
	</classpathentry>

首先跟踪 pom.xml 中的 Maven 编译器属性是如何被读取并转换为生成的 .classpath 文件中的 JRE 容器的。使用链接的示例 Maven 项目重现该问题，并检查不受支持的版本是否会选择最高的受支持 Java 版本，而不是 J2SE-1.5。