Fix Protobuf Version to 3.19.x (#88)

Protobuf 3.20 might be releasing soon (https://pypi.org/project/protobuf/3.19.3/#history). Fix Protobuf to 3.19 to be consistent with only allowing bug fixes similar to other packages.

tf_sig_build_dockerfiles/devel.requirements.txt

@@ -13,7 +13,7 @@ h5py ~= 3.6.0
 keras_preprocessing ~= 1.1.2
 numpy ~= 1.21.4
 opt_einsum ~= 3.3.0
-protobuf >= 3.17.1
+protobuf ~= 3.19.3
 six ~= 1.16.0
 termcolor ~= 1.1.0
 typing_extensions ~= 3.10.0.0