Why backup failed after Ansible Tower was upgraded from 3.5 on RHEL 8?

Issue

After upgrading Ansible Tower 3.5 on RHEL 8 with external DB to higher version, backup has been failed on the following TASK.

TASK [backup : Perform a PostgreSQL dump.] *************************************
skipping: [tower2] => {"changed": false, "skip_reason": "Conditional result was False"}
skipping: [tower3] => {"changed": false, "skip_reason": "Conditional result was False"}
fatal: [tower1]: FAILED! => {"changed": true, "cmd": " pg_dump  --clean --create  --host='postgres'  --port=5432  --username='awx'  --dbname='awx'  --exclude-table=main_instance  --exclude-table=main_instancegroup  --exclude-table=main_instancegroup_instances > tower.db\n", "delta": "0:00:00.022901", "end": "2020-05-22 01:18:54.539851", "msg": "non-zero return code", "rc": 1, "start": "2020-05-22 01:18:54.516950", "stderr": "pg_dump: server version: 10.6; pg_dump version: 9.6.10\npg_dump: aborting because of server version mismatch", "stderr_lines": ["pg_dump: server version: 10.6; pg_dump version: 9.6.10", "pg_dump: aborting because of server version mismatch"], "stdout": "", "stdout_lines": []}