Fix DTD usage of Checkstyle's import control

Author: snicoll

src/checkstyle/import-control.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0"?>
-<!DOCTYPE import-control PUBLIC "-//Puppy Crawl//DTD Import Control 1.1//EN" "http://www.puppycrawl.com/dtds/import_control_1_1.dtd">
+<!DOCTYPE import-control PUBLIC "-//Checkstyle//DTD ImportControl Configuration 1.4//EN" "https://checkstyle.org/dtds/import_control_1_4.dtd">
 <import-control pkg="org.springframework.boot">
 	<allow pkg=".*" regex="true" />