Add Protobuf (main, 33.x, 35.x) to downstream pipeline (#2808)

This PR configures Protobuf in the downstream pipeline:
- Adds Protobuf (main), Protobuf 33.x, and Protobuf 35.x to
`DOWNSTREAM_PROJECTS_PRODUCTION` using `.bazelci/presubmit.yml` from
`https://github.com/protocolbuffers/protobuf.git`.
- Removes obsolete `pipelines/protobuf.yml`.
- Supports `git_branch` in `DOWNSTREAM_PROJECTS` by passing
`git_commit="origin/<branch>"` to the downstream `project_pipeline` step
and skipping `get_last_green_commit` when a branch is configured.
- Adds unit tests in `buildkite/bazelci_test.py`.