In Groovy v2.x, the groovy.util.XmlParser was automatically imported as it was included in the default imports as above. Therefore you did not have to write import groovy.util.XmlParser explicitly.
When Katalon Studio was upgraded from v8.x to v9.0, they upgraded the bundled Groovy from v2 to v3. When Groovy upgraded from v2 to v3, the groovy.util.XmlParser was moved to groovy.xml.XmlParser.
In Groovy v3.x and newer, there is a class groovy.xml.XmlParser but the default imports do not include the class. Thefore you are required to write import groovy.xml.XmlParser explicitly.
Concerning this issue, Katalon has no problem, I believe.