build(deps-dev): bump google-protobuf in /demos/grpc-browser

Bumps [google-protobuf](https://github.com/protocolbuffers/protobuf) from 3.15.1 to 3.15.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.1...v3.15.6)

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

demos/grpc-browser/package.json

@@ -14,7 +14,7 @@
     "grpc-web": "^1.2.1"
   },
   "devDependencies": {
-    "google-protobuf": "^3.15.1",
+    "google-protobuf": "^3.15.6",
     "webpack": "^5.23.0",
     "webpack-cli": "^4.5.0",
     "webpack-dev-server": "^3.11.2"