Upgrade to Scala 2.13.6

[milanvdm]

I've built elastic/apm-agent-java#1918 locally and used it for your project.

From what I can tell, the post-requests are traced correctly.

The get-requests are not traced correctly due to the akka-actors. Ill check if it is still traced correctly when I remove the actors and use Futures instead.

Other apm tools specifically instrument akka-actors so I assume there is some context-switching happing there as well: https://github.com/kamon-io/Kamon/tree/master/instrumentation/kamon-akka/src/akka-2.6/scala/kamon/instrumentation/akka/instrumentations/akka_26

[henrikno]

Thank you for this!
I built elastic/apm-agent-java#1918 locally and built this PR, but I'm still seeing issues with the post request (which does use akka-actors).
First request two gives a warning about not properly deactivating spans

transaction-id: 3167cd8ed420110f Starting transaction 3167cd8ed420110f thread=204
transaction-id: 3167cd8ed420110f UserRegistry.CreateUser  thread=203
transaction-id: 3167cd8ed420110f Ending transaction 3167cd8ed420110f thread=203
2021-08-04 17:03:56,596 [HelloAkkaHttpServer-akka.actor.default-dispatcher-11] WARN  co.elastic.apm.agent.impl.ElasticApmTracer - Deactivating a span ('POST /users' 00-cf6d96595a05a738e8c672ec9e2e7692-3167cd8ed420110f-01 (3346b27d)) which is not the currently active span (null). This can happen when not properly deactivating a previous span.
ransaction-id: d49f322439f7ad06 Starting transaction d49f322439f7ad06 thread=258
transaction-id: d49f322439f7ad06 UserRegistry.CreateUser  thread=259
transaction-id: d49f322439f7ad06 Ending transaction d49f322439f7ad06 thread=262
2021-08-04 17:17:01,858 [HelloAkkaHttpServer-akka.actor.default-dispatcher-20] WARN  co.elastic.apm.agent.impl.ElasticApmTracer - Deactivating a span ('POST /users' 00-1c5eb2dd1e0ba3c26d9a4ce7d0c57711-d49f322439f7ad06-01 (1bd8499e)) which is not the currently active span (null). This can happen when not properly deactivating a previous span.

Third request didn't but the transaction id is wrong

transaction-id: d49f322439f7ad06 Starting transaction d3cd93f208a4e976 thread=258
transaction-id: d3cd93f208a4e976 UserRegistry.CreateUser  thread=273
transaction-id: d49f322439f7ad06 Ending transaction d3cd93f208a4e976 thread=258

I don't get how ElasticApm.currentTransaction().getId can give different results than transaction.getId right after running activate.
But I guess it's related to the earlier warnings.
Scope.activate() docs do say Note: activate() and Scope.close() have to be called on the same thread. but I don't get how we can do that on an async http server with a thread pool?
(Btw. I managed to get this Warn with just Future.map as well, although less often because it often happens to batch them on the same thread)