Upgrade to latest Checkstyle Suppressions DTD

See https://github.com/checkstyle/checkstyle/blob/master/config/suppressions.xml

src/checkstyle/checkstyle-suppressions.xml

 <?xml version="1.0"?>
-<!DOCTYPE suppressions PUBLIC
-    "-//Puppy Crawl//DTD Suppressions 1.1//EN"
-    "http://www.puppycrawl.com/dtds/suppressions_1_1.dtd">
+<!DOCTYPE suppressions PUBLIC "-//Checkstyle//DTD SuppressionFilter Configuration 1.2//EN" "https://checkstyle.org/dtds/suppressions_1_2.dtd">
 <suppressions>
 
 	<!-- global -->