Resolves [[ https://linear.app/comm/issue/ENG-5651/make-proto-field-names-underscore-separated-names | ENG-5651 ]].
Made our protos follow the [[ https://protobuf.dev/programming-guides/style/ | Style Guide ]].
WARNING: This is a breaking change! Fortunately, we don't use protos in prod yet, except keyserver cron job to update reserved usernames. Hopefully, format of that message hasn't been changed here, so at least in theory it should be fine.