chore: update protoc binary to v3.18.0 (#387)
chore: update protoc binary to v3.18.0 (#387)

Binary from https://github.com/protocolbuffers/protobuf/releases/download/v3.18.0/protoc-3.18.0-linux-x86_64.zip.

This binary is only used for loading integration test protos in CI.
googleapis/gapic-generator-php,googleapis/gapic-generator-php