PHPCS: set the supported PHP versions in the ruleset file

This commit moves the definition of the supported PHP versions from
RoboFile.php file to the PHPCS configuration file, ruleset.xml. This
should make it easier to configure PHPCS in other places like the IDE.

I also removed the constraint of the highest supported PHP version
(previously, it was hard-coded to 8.0) as I believe we support new PHP
versions as soon as they are released. With this change, we don't need to
remember to update testVersion tag whenever there is a new PHP version.

[MAILPOET-3439]

RoboFile.php

@@ -407,7 +407,6 @@ class RoboFile extends \Robo\Tasks {
 
     return $this
       ->taskExec($task)
-      ->rawArg('--runtime-set testVersion 7.1-8.0')
       ->arg('--ignore=' . implode(',', $foldersToIgnore))
       ->rawArg($stringFilesToCheck)
       ->run();