Polish "Use HTTPS for external links from XML files where possible"

See gh-16311

spring-boot-parent/src/checkstyle/checkstyle-suppressions.xml

 <?xml version="1.0"?>
 <!DOCTYPE suppressions PUBLIC
-    "-//Puppy Crawl//DTD Suppressions 1.1//EN"
-    "https://www.puppycrawl.com/dtds/suppressions_1_1.dtd">
+    "-//Checkstyle//DTD SuppressionFilter Configuration 1.2//EN"
+    "https://checkstyle.org/dtds/suppressions_1_2.dtd">
 <suppressions>
 	<suppress files="SpringApplicationTests\.java" checks="FinalClass" />
 	<suppress files=".+Configuration\.java" checks="HideUtilityClassConstructor" />

spring-boot-parent/src/checkstyle/checkstyle.xml

 <?xml version="1.0"?>
-<!DOCTYPE module PUBLIC "-//Puppy Crawl//DTD Check Configuration 1.2//EN" "https://www.puppycrawl.com/dtds/configuration_1_2.dtd">
+<!DOCTYPE module PUBLIC "-//Puppy Crawl//DTD Check Configuration 1.2//EN" "https://checkstyle.org/dtds/configuration_1_2.dtd">
 <module name="Checker">
 
 	<!-- Root Checks -->