Classify XML as a data language

While XML is technically a markup language, in the majority of cases it
is just a serialization format for a tool (e.g., project files for IDEs)
rather than hand-authored markup. As such it isn't really useful to
include it in repository language statistics. A C# project doesn't
really care whether Visual Studio uses XML, JSON, or some other format
to serialize its project files, for example.

lib/linguist/languages.yml

@@ -3165,7 +3165,7 @@ XC:
   ace_mode: c_cpp
 
 XML:
-  type: markup
+  type: data
   ace_mode: xml
   aliases:
   - rss