That's a php 7.4 warning. The extension org.cpehn.subscriptionhistory needs to make the change in the extension and then they should release a new version. I would post an issue there: https://github.com/MegaphoneJon/org.cpehn.subscriptionhistory/issues
Welcome to SE! Can you install cv? If so, I find that the most straightforward for cron. See https://docs.civicrm.org/sysadmin/en/latest/setup/jobs/#cv
For the HTTP method, does the user/password work when logging in via GUI? Does it work with curl or wget? (See same doc) Check that works before adding the complications of cron itself.