How to prevent the owner of the test case to approve their own test case ?

If you want peer-approval, the said approach does not work well, because there has to be an "approver" role, which is different from the test case owner's role. I have come up with this idea, and only you can decide whether it's "better". I assume the testers who create test cases and approve the test cases have the same role (peer approval), let's say the role is just "Testers".
1. Modify the Require Review precondition so that the approval requires at least _two_ approvers from the Testers role.
2. Revoke the permission Quality Management > Save Test Cases > Trigger a workflow action > Approve from the role "Testers".
3. Set up a Test Artifact Approval Tracking for the Test Case workflow so that an Approved Approval uses the Approve action to move the workflow state from Under Review to Approved.

And that's it. Step 3 will automatically change any test cases with an approved approval to the Approved state. Step 2 is to prevent Testers from moving test cases to the Approved state directly. Step 1 is to prevent the Testers from setting himself to the Approval and subsequently approve the approval (in order to approve the test case indirectly).

You can give the permission to other roles to directly move the test case state to Approved, if required.