Update protobuf requirement from ~=4.21.7 to ~=4.21.8

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/generate_changelog.py)
- [Commits](https://github.com/protocolbuffers/protobuf/commits)

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

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

requirements.txt

@@ -89,7 +89,7 @@ pluggy~=1.0.0
 portalocker~=2.5.1
 prometheus-client~=0.15.0
 prompt-toolkit~=3.0.31
-protobuf~=4.21.7
+protobuf~=4.21.8
 psutil~=5.9.2
 pure-eval~=0.2.2
 py~=1.11.0