consider changes to pekko-protobuf-v3

This is different to #239 - which is about pekko-protobuf as opposed to pekko-protobuf-v3.

pekko-protobuf-v3 is built by taking protobuf-java and shading it. It's wasteful because we create separate jars from Scala 2.12, 2.13 and 3 - there is no Scala code, it is just Java.

In v1.1.0, we could change our code.
* we could just use protobuf-java directly - I know some users won't be happy because they use Pekko and some other code that uses an incompatible version of protobuf-java - we could argue that this is not our concern
* we could just publish one pekko-protobuf-v3 jar that has no Scala version suffix