Fix migration source type

Previously the migration-7/web.adoc source type was kotlin for the xml
source. This commit fixes that.

Closes gh-16662

docs/modules/ROOT/pages/migration-7/web.adoc

@@ -91,7 +91,7 @@ http {
 
 Xml::
 +
-[source,kotlin,role="secondary"]
+[source,xml,role="secondary"]
 ----
 <http entry-point-ref="myEntryPoint">
     <!-- ... -->