Update codegen/README.md to match removal of protobuf 3 code

Author: gabrys

codegen/README.md

@@ -23,29 +23,8 @@ This will output the code in `src/` at the root of the repository.
 ./generate-proto.sh
 ```
 
-This command builds and runs two separate docker containers that generate protobuf code
-using toolchains compatible with protobuf==3.19.6 and protobuf==5.29.4 (effectively protobuf 4+).
+This command builds and runs a Docker container that generates protobuf code
+using toolchains compatible with protobuf==5.29.4 (effectively protobuf 4+).
 
 We run the generation tools in a docker container, to ensure specific versions: grpcio-tools
 and Python in particular.
-
-## Compatibility layer
-
-The generated code lands in the following directories:
-
-Protobuf 3: `src/neptune_api/proto/protobuf_v3`
-Protobuf 4+: `src/neptune_api/proto/protobuf_v4plus`
-
-There is a simple compatibility layer in `neptune_api/proto/__init__.py`  that loads the
-correct module based on the installed `protobuf` version.
-
-The `.pyi` files in the `proto` directories use the v3 versions, to minimize potential
-compatibility risks. It is an arbitrary choice. You can point your IDE to the v4+
-version if you prefer.
-
-# Why keep two versions?
-
-* We want to have as broad range of compatibility as possible.
-* Version 4+ uses upb, which is a more efficient serialization format.
-* We cannot be sure whether the code generated by old grpcio will always take advantage
-  of changes introduced in newer protobuf versions.