Catch up on changes in protobuf up to v28.0.

[chungyc]

• Update location to the current one at https://github.com/protocolbuffers/protobuf
• Update protobuf submodule to v28.0
• Regenerate bootstrapping protobuf bindings with runghc bootstrap.hs

This is preparatory work for supporting Protobuf Editions (issue #468), which requires the updates to the protocol buffer message types defined for protoc plugins.

[blackgnezdo]

Looks good, thanks @chungyc