Fix domtrip API breakage after 1.5.1 upgrade (#12139)

The children(String) method was renamed to childElements(String)
in domtrip 1.5.1, causing a compilation failure.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: gnodet

impl/maven-cli/src/main/java/org/apache/maven/cling/invoker/mvnup/goals/AbstractUpgradeGoal.java

@@ -310,7 +310,7 @@ protected void fixIncompatibleExtensions(UpgradeContext context) {
             boolean modified = false;
             boolean needsNisseCompat = false;
 
-            List<Element> extensions = root.children("extension").toList();
+            List<Element> extensions = root.childElements("extension").toList();
             List<Element> toRemove = new ArrayList<>();
 
             for (Element ext : extensions) {