Why the task fails to install postgresql-upgrade96 when upgrading from Ansible Tower 3.5 on RHEL8 with external DB?

Issue

When running setup.sh for upgrading Ansible Tower from 3.5 on RHEL 8 with external DB, the following task will be failed.

TASK [packages_el : Install Postgresql Upgrade packages [RHEL 8]] ***************************************************
fatal: [postgres]: FAILED! => {"changed": false, "failures": ["No package postgresql-upgrade96 available."], "msg": "Failed to install some of the specified packages", "rc": 1, "results": []}