Require process-bundle 5.5, remove the compatibility code of #220 #226

#220 (PR #221) records a process stopped by a StopTask with the stopped status through ProcessEvent::isProcessStopped(), added by cleverage/process-bundle 5.5 (cleverage/process-bundle#337). Until this release, the bundle kept a compatibility code with process-bundle 5.4:

• method_exists() in ProcessEventSubscriber::isStoppedOnPurpose();
• the skip of ProcessEventSubscriberTest::testProcessStoppedByAStopTask();
• 3 ignored errors in phpstan.neon.

process-bundle 5.5 is released: require cleverage/process-bundle ^5.5 and remove this compatibility code.