Use native scala 3 akka artifacts.

[cptwunderlich]

Upgrade to latest akka 2.6 patch version (2.6.21) and use native Scala 3 artifacts, instead of 2.13 cross-versions. These are binary imcompatible, at least with later akka versions on Scala 3.
Fixes issue #298

[cptwunderlich]

OK; so the run fails bc. it couldn't find sbt? Strange.
However, the versions of the actions are quite old (v2 vs v4). Also, why use Java 15 and not 11 or 17 LTS?
Also, the Pull workflow differs from the main one. This one still tests with Akka 2.5, this is probably not what we want, right? I'm saying that bc. of the support table in the README. Otherwse I'll have to change the build.sbt file, bc. there are no Scala 3 native releases for Akka 2.5

[cptwunderlich]

I updated the Github actions, removed Akka 2.5 tests, used Java 17 and switched to "temurin" instead of "adopt" (there is no adoptium distribution anymore, it's temurin now).
If those changes are too much, I can open a separate PR, but I'd like to see if this fixes the issue.

[cptwunderlich]

OK; no, apparently, the Ubuntu image was changed to 24.04, which doesn't include sbt and it broke for everyone: actions/setup-java#712

[danischroeter]

Hi @cptwunderlich
We have internally discussed how to proceed properly with this project. Please see #300
We appreciate your effort in this PR - but as it currently stands we will not be able to merge it and create a release.
This can of course be discussed with a future maintainer of the project - maybe you are interested?