Update install_et.sh to install numpy 1.21.3

Author: Jack-Khuu

torchchat/utils/scripts/install_et.sh

@@ -24,5 +24,5 @@ install_executorch_python_libs $ENABLE_ET_PYBIND
 # has no attribute 'utils'' error from evaluate CI jobs and remove
 # `import lm_eval` from torchchat.py since it requires a specific version
 # of numpy.
-pip install numpy=='1.26.4'
+pip install numpy=='1.21.3'
 popd