Update protobuf requirement from <6,>=4.21.6 to >=4.21.6,<7

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@v4.22.0-rc1...v6.30.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

pyproject.toml

@@ -40,7 +40,7 @@ dependencies = [
   "frequenz-channels >= 1.0.0-rc1, < 2.0.0",
   "frequenz-client-base >= 0.8.0, < 0.10.0",
   "grpcio >= 1.54.2, < 2",
-  "protobuf >= 4.21.6, < 6",
+  "protobuf >= 4.21.6, < 7",
   "timezonefinder >= 6.2.0, < 7",
   "typing-extensions >= 4.5.0, < 5",
 ]