pip: update protobuf requirement from <7 to <8

Updates the requirements on [protobuf](https://github.com/protocolbuffers/protobuf) to permit the latest version.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Commits](https://github.com/protocolbuffers/protobuf/commits)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-version: 7.35.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

tests/requirements.txt

@@ -19,7 +19,7 @@ ray[serve]==2.56.1
 # `AttributeError: 'google._upb._message.FieldDescriptor' object has no
 # attribute 'label'` in test_rayserve_remote_task_multiple_concurrent.
 # Pin to 6.x until Ray ships a fix.
-protobuf<7
+protobuf<8
 ipykernel==7.3.0
 jupyter_book==2.1.6
 pytest-xdist==3.8.0