fix(ci): pin protobuf <4 for legacy descriptor-style pb2 files

Generated pb2 files use old-style FieldDescriptor() calls that are
incompatible with protobuf >=4. Pin to 3.20.x range to match the
firmware Docker container's protobuf version.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: BitHighlander

.github/workflows/ci.yml

@@ -71,6 +71,7 @@ jobs:
       - name: Install dependencies
         run: |
           pip install --upgrade pip
+          pip install "protobuf>=3.20,<4"
           pip install -e .
           pip install pytest semver rlp