Add Akka resolver so I can unfreeze some downstream-from-Akka repos

[SethTisue]

in #1703 I froze some repos because they started depending on an Akka milestone version not available from Maven Central

according to internal email,

We have [...] moved the downloads to our own repository on Cloudsmith

in the short term it's okay to leave the projects frozen, but it would be good to add the resolver to our Artifactory eventually

resolver is https://repo.akka.io/maven

note that the new Akka versions dropped JDK 8 support, so I'll need to update the report script for that