Update the user-agent used in cronjob GET requests to be descriptive

Helmorequested to merge
helmo/ispconfig3:6944-cron-user-agent-wget into ispconfig-3-3-1

Issue #4020 (closed) needed something else then the default from wget, I assume because some system there did not like wget accessing the site. However such an old Firefox string is just misleading.

Closes #6944 (closed)

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