Relax CI pin on Aer <0.13 (#11313)

Now that Aer 0.13.1 is released, we expect that all the bugs and issues
we had Qiskit-side are resolved.  This releases the pin on the lower
version of Aer to get us testing properly in CI again.

(cherry picked from commit 8b38060)

constraints.txt

@@ -3,11 +3,6 @@
 # https://github.com/Qiskit/qiskit-terra/issues/10345 for current details.
 scipy<1.11; python_version<'3.12'
 
-# 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
-
 # z3-solver from 4.12.3 onwards upped the minimum macOS API version for its
 # wheels to 11.7. The Azure VM images contain pre-built CPythons, of which at
 # least CPython 3.8 was compiled for an older macOS, so does not match a