Update backend() type hint to BackendV2 (Qiskit#1442)

Co-authored-by: Jessie Yu <[email protected]>

qiskit_ibm_runtime/qiskit_runtime_service.py

@@ -20,7 +20,7 @@
 from collections import OrderedDict
 from typing import Dict, Callable, Optional, Union, List, Any, Type, Sequence
 
-from qiskit.providers.backend import BackendV1 as Backend
+from qiskit.providers.backend import BackendV2 as Backend
 from qiskit.providers.provider import ProviderV1 as Provider
 from qiskit.providers.exceptions import QiskitBackendNotFoundError
 from qiskit.providers.providerutils import filter_backends