Remove FakeBackendV2 import deprecated in Qiskit 0.46 (#1420)

### Summary

In qiskit 0.46, several classes in the `qiskit.providers.fake_provider`
module were deprecated, including
`qiskit.providers.fake_provider.fake_backend.FakeBackendV2`. However,
the removal PR failed to address this particular class, so while this
particular `FakeBackendV2` was no longer part of the public API, the
import path would still work. This oversight will be addressed in
Qiskit/qiskit#11997, but in order to merge this
PR we must make sure that all the downstream libraries covered by
`qiskit-neko` are up-to-date with the change.

This change adds an alternative import path for this class in
qiskit-experiments. It follows the pattern from other 1.0 import
deprecations. The removal would roll out in the 1.1 release, but it is
indicated as a 1.0 removal because that is when it should have
technically taken place.

### Details and comments

Blocks Qiskit/qiskit#11997.

Author: ElePT

qiskit_experiments/framework/backend_data.py

@@ -18,9 +18,24 @@ class unifies data access for various data fields.
 from qiskit.providers.models import PulseBackendConfiguration
 from qiskit.providers import BackendV1, BackendV2
 from qiskit.providers.fake_provider import FakeBackend
-from qiskit.providers.fake_provider.fake_backend import FakeBackendV2
 from qiskit.utils.deprecation import deprecate_func
 
+try:
+    # Removed in Qiskit 1.0.
+    from qiskit.providers.fake_provider.fake_backend import FakeBackendV2
+except ImportError:
+
+    class FakeBackendV2:
+        """Dummy class for when FakeBackendV2 import fails
+
+        This class is only used in isinstance checks. If the import fails, then
+        there won't be an instance of the class either so any dummy class is
+        fine.
+        """
+
+        pass
+
+
 try:
     # Removed in Qiskit 1.0. Different from the other FakeBackendV2's
     from qiskit.providers.fake_provider import QiskitFakeBackendV2