Update suppressions config (smithy-lang#274)

Author: Chase Coalwell

config/checkstyle/suppressions.xml

@@ -15,7 +15,7 @@
   -->
 
 <!DOCTYPE suppressions PUBLIC
-        "-//Puppy Crawl//DTD Suppressions 1.1//EN"
-        "http://www.puppycrawl.com/dtds/suppressions_1_1.dtd">
+        "-//Checkstyle//DTD SuppressionFilter Configuration 1.2//EN"
+        "https://checkstyle.org/dtds/suppressions_1_2.dtd">
 <suppressions>
 </suppressions>