[checkstyle] Switch to new DTD (mockito#1940)

The location for the checkstyle DTD has moved.

config/checkstyle/checkstyle.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0"?>
 <!DOCTYPE module PUBLIC
     "-//Puppy Crawl//DTD Check Configuration 1.2//EN"
-    "http://www.puppycrawl.com/dtds/configuration_1_2.dtd">
+    "https://checkstyle.org/dtds/configuration_1_2.dtd">
 <module name="Checker">
     <!--
      Different editors and IDEs may insert or remove trailing