Update protobuf version for tf-keras.

PiperOrigin-RevId: 775884932

Author: hertschuh

requirements.txt

@@ -4,11 +4,11 @@ pandas
 pydot
 scipy
 # Remove once both TensorFlow and TF-Keras nightly builds pass.
-# Temporarily enforce 4.21.6 version, as the only version which is compatible
+# Temporarily enforce 5.28.0 version, as the only version which is compatible
 # with both new and old protobuf stubs. This is needed to resolve
 # Keras-TensorFlow circular dependency issue, when one of them gets a dependency
 # incompatible with another one (protobuf in this specific case).
-protobuf==4.21.6
+protobuf==5.28.0
 tf-nightly
 portpicker
 pyyaml