Payment Processor Error message 9003: MD5 Verification failed after upgrade
I found the issue. The ca.civicrm.moneris extension folder was deleted during the drush civicrm-upgrade process.

As a result, my "Moneris Credit Card" Payment Processor Type disappeared and defaulted to Authorize.net (which uses an MD5 hash algorithm) after the upgrade. After restoring this, I entered my API token as opposed to the store admin password in the Payment Processor Edit form and my transactions are now working again. I have renamed this field label to "API Token."