Constrain qiskit-aer==0.12.2 (Qiskit#11120)

The 0.13.0 release of qiskit-aer causes several test failures, some of
which look like changes in the randomisation, and some of which look
like true failures.  These need a proper resolution, but it's more work
than can be easily attached to a PR that's just meant to get CI rolling
again.

constraints.txt

@@ -11,3 +11,8 @@ numpy<1.25
 # eigensystem code for one of the test cases.  See
 # https://github.com/Qiskit/qiskit-terra/issues/10345 for current details.
 scipy<1.11
+
+# Aer 0.13 causes several randomised tests to begin failing, and some
+# `QuantumInstance` use of noise models to raise exceptions.  These need fixes
+# on Terra.
+qiskit-aer==0.12.2