Update protobuf-java to 3.15.8

[scala-steward]

Updates com.google.protobuf:protobuf-java from 3.15.6 to 3.15.8. GitHub Release Notes - Version Diff

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labels: library-update, semver-patch

[sequencer]

sbt is still using protocjar from os72, which seems not being actively maintained anymore.

lazy val protobufSettings = Seq(
  sourceDirectory in ProtobufConfig := baseDirectory.value / "src" / "main" / "proto",
  protobufRunProtoc in ProtobufConfig := (args =>
    com.github.os72.protocjar.Protoc.runProtoc("-v351" +: args.toArray)
  )
)

How should we resolve this? I think writing sbt script like what I did in mill should be a good idea, but I don't know how to get sbt work.