Pin Numpy < 2 in requirements

We have compiled extensions that are built against the Numpy C API.  The
Numpy transition guide for developers of downstream packages[^1]
encourages us to put in this pin until we have wheels built against the
newer version, which is not expected to be fully ABI compatible (but
extensions built against Numpy 2 _should_ work with older Numpys).

Note that this won't prevent package managers from resolving _older_
versions of Qiskit (which don't have the pin) along with Numpy 2, but
there's not a vast amount we can do about that now.

[^1]: numpy/numpy#24300

releasenotes/notes/numpy-2-pin-f600328f24fab6dd.yaml

@@ -0,0 +1,10 @@
+---
+other:
+  - |
+    This version of Qiskit is explicitly pinned to the Numpy 1.x series, because it includes compiled
+    extensions that are not yet compiled against the as-yet-unreleased Numpy 2.x series.  We will
+    release a new version of Qiskit with Numpy 2.x support as soon as feasible.
+
+    We cannot prevent your package manager from resolving to older versions of Qiskit (which do not
+    have the same pin but are still likely to be incompatible) if you forcibly try to install Qiskit
+    alongside Numpy 2, before we have released a compatible version.

requirements.txt

@@ -1,5 +1,5 @@
 rustworkx>=0.13.0
-numpy>=1.17
+numpy>=1.17,<2
 ply>=3.10
 psutil>=5
 scipy>=1.5