MNT: (deps): Bump protobuf from 3.15.6 to 3.15.7 in /ci
MNT: (deps): Bump protobuf from 3.15.6 to 3.15.7 in /ci

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.15.6 to 3.15.7.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](https://github.com/protocolbuffers/protobuf/compare/v3.15.6...v3.15.7)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
Unidata/siphon