Use BackendV2 for basic aer

[mtreinish]

What should we add?

Right now the backends in qiskit.providers.basicaer are all built on the legacy BackendV1 model. We should update it to be based on the more modern BackendV2 instead. The target will be very simple for these backends, it's basically just:

MAX_QUBITS_MEMORY = int(log2(local_hardware_info()["memory"] * (1024**3) / 16))
target = Target(num_qubits=min(24, MAX_QUBITS_MEMORY)
lam = Parameter("lambda")
target.add_instruction(U1Gate(lam))
phi = Parameter("phi")
target.add_instruction(U2Gate(phi, lam))
theta = Parameter("theta")
target.add_instruction(U3Gate(theta, phi, lam))
target.add_instruction(RZGate(phi))
target.add_instruction(SXGate())
target.add_instruction(XGate())
target.add_instruction(CXGate())
target.add_instruction(IGate())
target.add_instruction(UnitaryGate, name="unitary")

The trickier piece here is more about maintaining backwards compatibility for all the pieces that are part of the BackendV1 interface and not in the BackendV2 interface to give users a chance to migrate.

[burgholzer]

Might I point out #10592 in that regard as it also popped up during an attempt to move our classical simulation backends from BackendV1 to BackendV2?

[MozammilQ]

@mtreinish , am I allowed to try this one?