Use sbt-rsync plugin instead of github action or remove sbt-publish-rsync

While looking at https://github.com/apache/incubator-pekko/issues/103 I noticed that the pekko project already happens to have infrastructure setup to publish nightlies (aka snapshots) to an rsync directory in the exact same manner that was implemented in https://github.com/apache/incubator-pekko/pull/60.

This is done by using https://github.com/lightbend/sbt-publish-rsync and you can see the details at https://github.com/apache/incubator-pekko/blob/main/project/Publish.scala.

We should either investigate using the sbt-publish-rsync plugin or remove it entirely from pekko since https://github.com/apache/incubator-pekko/blob/main/.github/actions/sync-nightlies/action.yml exists. My own personal preference would be to use sbt-publish-rsync since its far simpler (due to it being done by sbt the command to publish would be quite trivial especially when combined with publishing to Maven snapshots directory, see https://github.com/apache/incubator-pekko/issues/103) and I like reducing the direct dependance on shell scripts. From a cursory glance of looking at https://github.com/lightbend/sbt-publish-rsync/blob/master/src/main/scala/com/lightbend/sbt/publishrsync/PublishRsyncPlugin.scala#L45-L49 it shouldn't be problematic however there is a risk that changes might need to be done to sbt-publish-rsync plugin for Apache's use, if needed this does present an opportunity to create an sbt-publish-rsync-apache plugin.

@seglo @pjfanning Thoughts?