* Move proto to OTLP exporter
* Update OTLP exporter import of proto
* Use gogo protobuf
To stop using the deprecated github.com/golang/protobuf and match what
the collector is doing, switch to generating OTLP with the
github.com/gogo/protobuf/proto instead of
github.com/golang/protobuf/proto.
* Clean dependencies
Remove all protobuf dependencies from otel package and all of its
dependencies.
* Update CHANGELOG
* Clean OTLP exporter go mod
Remove submodule beforehand to avoid unneeded direct dependencies.
* Use default ref for GitHub workflow
* Update path that triggers proto gen action
* Correct license-check exclusion for OTLP
* Update commented location of the OTLP and code
* Import open-telemetry/opentelemetry-proto submodule under internal
* Commit changes in updated/new protobuf files
* Refer to new location of .pb.go files after rewrite from import
* Describe in CHANGELOG
