UPDATE: I was able to manually work around the issue.

1. Run terraform apply
2. After apply fails, copy the IAM role ARN and name before deleting the storage credential with the incorrect External ID
3. Manually create the storage credential using the same IAM role and name copied from the storage credential deleted in the previous step
4. Re-run terraform plan
5. Re-run terraform apply

While this is workable, I don't consider it a permanent solution and I would still appreciate a solution that will remove the need for a manual work around.