plugin converters - javaparser error with Xml.java

[jdaugherty]

Issue description

From the initial M2 publish:

> Task :grails-plugin-validation:publishToMavenLocal
Attempting to ignore error parsing Java source file: grails/converters/XML.java
Consider reporting the error to the Groovy project: https://issues.apache.org/jira/browse/GROOVY
... or directly to the JavaParser project: https://github.com/javaparser/javaparser/issues
Error: (line 151,col 41) Use of patterns with instanceof is not supported. Pay attention that this feature is supported starting from 'JAVA_14' language level. If you need that feature the language level must be configured in the configuration before parsing the source files.