Update conditional tests once `c_if` support is dropped in Aer

[ElePT]

What is the expected behavior?

Qiskit 2.0 removed c_if, but this is still supported in Aer during the transition period to 2.0. As a consequence, there is a series of unit tests that will get skipped if qiskit>=2.0. Once Aer drops support c_if, these tests should be refactored to test the conditional syntax that uses IfElseOp.

See https://github.com/Qiskit/qiskit-aer/pull/2327#discussion_r1993022971.