=doc switch to GitHub Issues

also remove the `experimental` marker from multi-node-testing’s docs
=pro Update genjavadoc version to 0.7
Merge pull request #15005 from akka/wip-3969-update-gen-javadoc-ban

=pro Update genjavadoc version to 0.7
=con #3993 Send UnsubscribeAck in DistributedPubSubMediator to the right sender

(cherry picked from commit d9f0a1aac3c332d2e4767d0a478310c011f86b9a)
Merge pull request #15014 from akka/wip-3993-backport-patriknw

=con #3993 Send UnsubscribeAck to the right sender (for validation)
=pro Update test dependencies to Scala 2.11 cross published versions