Added Scala 3 support

[AndreasKillaitis0000]

The library can now be cross-built with Scala 2.12, 2.13 and 3.1. To achieve this, some modifications had to be made:

• update sbt version
• update all Scala dependencies to latest versions (especially Akka)
• update Specs2 Acceptance tests to recent syntax

Additionally, some return types have been made explicit

[enragedginger]

LGTM. Thanks for doing this. I'll try to get a new build out this week!