Replace deprecated HTTP URL references with HTTPS

CONTRIBUTING.md

@@ -31,10 +31,10 @@ then this style will be used automatically.
 
 Moreover, (since this project is about static code analysis :wink:) a configuration for the following static code
 analysis tools is defined in the POM (and the `etc` and `.idea` folders of the `codingstyle` module):
-- [Checkstyle](http://checkstyle.sourceforge.net/)
+- [Checkstyle](https://checkstyle.sourceforge.net/)
 - [PMD](https://pmd.github.io/)
 - [SpotBugs](https://spotbugs.github.io)
-- [Error Prone](http://errorprone.info)
+- [Error Prone](https://errorprone.info)
 - [IntelliJ](https://www.jetbrains.com/help/idea/code-inspection.html)
 
 This configuration will be picked up automatically if you build the project using Maven. If you install the CheckStyle