Bump protobuf in /examples/reservations-bot-demo/cloud_function

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.13.0 to 3.15.0.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.13.0...v3.15.0)

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

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

Author: dependabot[bot]

examples/reservations-bot-demo/cloud_function/requirements.txt

@@ -6,7 +6,7 @@ enum-compat==0.0.3
 googleapis-common-protos==1.52.0
 grpcio==1.31.0
 idna==2.10
-protobuf==3.13.0
+protobuf==3.15.0
 pycparser==2.20
 pydantic==1.6.2
 PyJWT==1.7.1