Update test_265_config_retpoline in qa-regression-testing for kernel config change

In test_265_config_retpoline from ubuntu_qrt_kernel_security, it's expecting to see CONFIG_RETPOLINE unless kernel is newer than 6.9:

        elif self.kernel_at_least('6.9'):
            # RETPOLINE was renamed into MITIGATION_RETPOLINE since v6.9
            # commit aefb2f2e61 upstream
            self.reportConfig('RETPOLINE', "RETPOLINE config was renamed into MITIGATION_RETPOLINE since v6.9")
            return

        self.assertKernelConfig('RETPOLINE', expected)

However this change, RETPOLINE was renamed into MITIGATION_RETPOLINE, has been backported to our Noble 6.8 kernels recently LP: #2144730

The test case needs to be updated accordingly.