Deprecate proto2 version of our artifacts

[alecgrieser]

At this point, we've been publishing dual proto2 and proto3 versions of the Record Layer for a while. This complicates our builds in a number of ways. We should be able to deprecate the separate builds and consolidate on a single protobuf version (version 3). The serialized data from both versions are binary compatible, so data written using the proto2 version of the Record Layer can be read from a proto3 version, so the only hurdle to adopters upgrading is if they have dependencies in their chain that require proto2 somehow. They should be able to use the shaded Jars, but giving them a heads up by deprecating the artifacts first seems like a good idea. I'm proposing we do that for 3.2, which would allow us to remove them entirely at a later release