Bump protobuf (#271)

## What is the goal of this PR?

We've bumped our protobuf dependency in response to the following issue:
- #270

## What are the changes implemented in this PR?

We've changed our pinned protobuf version from 3.15.5 to 3.20.1.

Author: James Williams

requirements.txt

@@ -33,5 +33,5 @@
 
 # IMPORTANT: Any changes to these dependencies should be copied to requirements_dev.txt.
 typedb-protocol==2.12.0
-grpcio==1.43.0
-protobuf==3.15.5
+grpcio>=1.43.0,<=1.44.0
+protobuf>=3.15.5,<=3.20.1

requirements_dev.txt

@@ -31,8 +31,8 @@
 ## Dependencies
 
 typedb-protocol==2.12.0
-grpcio==1.43.0
-protobuf==3.15.5
+grpcio>=1.43.0,<=1.44.0
+protobuf>=3.15.5,<=3.20.1
 
 
 # Dev dependencies (not required to use the typedb-client package, but necessary for its development)