Update protobuf requirement from <4,>3.6.1 to >3.6.1,<6

Updates the requirements on [protobuf](https://github.com/protocolbuffers/protobuf) to permit the latest version.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/protobuf_release.bzl)
- [Commits](protocolbuffers/protobuf@v3.6.1.1...v5.27.2)

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

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

requirements/test.txt

@@ -10,7 +10,7 @@
 -r storage/async-redis.txt
 
 # Pin to < 4 for aetcd compatibility
-protobuf>3.6.1,<4
+protobuf>3.6.1,<6
 
 # Test related packages
 coverage