Follow upstream master changes: edit addresses of Checkstyle DTD files (

#86)

Upstream se-edu/addressbook-level4@e09826e updated the addresses of
Checkstyle DTD files for security reason.

Let's follow and merge the changes from se-edu#984 into master branch.

config/checkstyle/checkstyle.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!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">
 
 <!--
     This configuration file enforces rules for a modified version of the module's code standard at

config/checkstyle/suppressions.xml

@@ -1,8 +1,8 @@
 <?xml version="1.0"?>
 
 <!DOCTYPE suppressions PUBLIC
-"-//Puppy Crawl//DTD Suppressions 1.1//EN"
-"http://checkstyle.sourceforge.net/dtds/suppressions_1_1.dtd">
+    "-//Checkstyle//DTD SuppressionFilter Configuration 1.2//EN"
+    "https://checkstyle.org/dtds/suppressions_1_2.dtd">
 
 <suppressions>
   <suppress checks="JavadocType" files=".*Test\.java"/>