Add release note

releasenotes/notes/plugable-rb-clifford-synthesis-0e66c62fa3088fba.yaml

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+---
+features:
+  - |
+    Added a new experiment option ``clifford_synthesis_method`` to RB experiemnts,
+    e.g. :class:`~.StandardRB`, :class:`~.InterleavedRB` so that users can
+    plug in a custom Clifford synsthesis algorithm used for generating RB circuits.
+    Such a plugin should be implemented as a ``HighLevelSynthesisPlugin``
+    (see :class:`~.RBDefaultCliffordSynthesis` for example).
+upgrade:
+  - |
+    Updated :class:`~.InterleavedRB` so that it only accepts ``interleaved_element``
+    consisting only of instructions supported by the backend of interest.
+fixes:
+  - |
+    Fixed a bug in circuit generation for three or more qubit RB where
+    sampled Cliffords may be changed during their circuits synthesis
+    (in the worst case, the resulting circuits may use qubits not in
+    ``physical_qubits``). See issue
+    `#1279 <https://github.com/Qiskit-Extensions/qiskit-experiments/issues/1279>`_
+    for additional details.