Update requirements.txt (#18382)

Update the tf version used for keras 2.14 rc.

The pip package verification test was incorrectly using the tf-nightly and cause the build to fail.

Author: qlzh727

requirements.txt

@@ -9,11 +9,11 @@ scipy ~= 1.9.2
 # Keras-TensorFlow circular dependency issue, when one of them gets a dependency
 # incompatible with another one (protobuf in this specific case).
 protobuf==3.20.3
-tf-nightly
+tensorflow==2.14.0rc1
 portpicker
 pyyaml
 Pillow
 numpy ~= 1.24.3  # Sync with the numpy version used in TF
 black==22.3.0
 isort==5.10.1
-flake8==4.0.1
+flake8==4.0.1