Update Checkstyle DTD location

The sourceforge location is hidden, the updated location is
on https and recommended.

This issue goes into more details on the DTD migration:
checkstyle/checkstyle#6478

config/checkstyle/checkstyle.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0"?>
 <!DOCTYPE module PUBLIC
           "-//Puppy Crawl//DTD Check Configuration 1.3//EN"
-          "http://checkstyle.sourceforge.net/dtds/configuration_1_3.dtd">
+          "https://checkstyle.org/dtds/configuration_1_3.dtd">
 
 <!--
     Checkstyle configuration that checks the Google coding conventions from Google Java Style