Your IT Department is absolutely, undeniably, wrong.
If something is not listed on the "Supported Platforms" page for your version, then it is not supported. OpenJDK is clearly not listed on https://confluence.atlassian.com/adminjiraserver073/supported-platforms-861253018.html therefore it is not supported.
I can also tell you that it doesn't work - Jira will start and run, but throw certain errors and gradually fail (more and faster if certain add-ons are installed)
...PermissionsStartOnly=true User=www-data Group=www-data ExecStart=/opt/jira/bin/startup.sh ExecStop=/opt/jira/bin/shutdown.sh TimeoutStartSec=120 TimeoutStopSec=600 PrivateTmp=true [Install] WantedBy...
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