fix(maven): loosen check for correct pom.xml

Previously Renovate checked for xmlns="http://maven.apache.org/POM/4.0.0" and if not found then did not attempt to parse the file. Instead, now it checks for `<modelVersion>4.0.0</modelVersion>` as an alternative. Reference: https://maven.apache.org/guides/introduction/introduction-to-the-pom.html

Closes #5160