Update Checkstyle DTDs links.

Checkstyle DTDs on puppycrawl site were deprecated some time ago and are now removed.
- Change configuration DTD link to "https://checkstyle.org/dtds/configuration_1_3.dtd"
- Change suppressions DTD link to "https://checkstyle.org/dtds/suppressions_1_2.dtd"

For more information see the following issues:
- checkstyle/checkstyle#1571
- checkstyle/checkstyle#6478

project_conf/checks.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0"?>
 <!DOCTYPE module PUBLIC
-  "-//Puppy Crawl//DTD Check Configuration 1.3//EN"
-  "http://www.puppycrawl.com/dtds/configuration_1_3.dtd">
+    "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+    "https://checkstyle.org/dtds/configuration_1_3.dtd">
 
 <!--
 

project_conf/suppression.xml

@@ -1,8 +1,8 @@
 <?xml version="1.0"?>
 
 <!DOCTYPE suppressions PUBLIC
-    "-//Puppy Crawl//DTD Suppressions 1.2//EN"
-    "http://checkstyle.sourceforge.net/dtds/suppressions_1_2.dtd">
+    "-//Checkstyle//DTD SuppressionFilter Configuration 1.2//EN"
+    "https://checkstyle.org/dtds/suppressions_1_2.dtd">
 
 <suppressions>
     <suppress files="[\\/]src[\\/]test[\\/].*" id="JavadocTypeMain" />