[0.11.x] Prepare Scala 3 (partly backport #488) by @mkurz

[mergify]

This is an automatic backport of pull request #488 done by Mergify.
Cherry-pick of bed3439 has failed:

On branch mergify/bp/0.11.x/pr-488
Your branch is ahead of 'origin/0.11.x' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit bed3439.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   play-interop-test-java/src/test/scala/play/grpc/PlayJavaRouterSpec.scala
	modified:   play-runtime/src/main/scala/play/grpc/internal/AkkaGrpcClientFactory.scala
	modified:   play-scalatest/src/test/scala/play/grpc/scalatest/PlayActionsScalaTestSpec.scala
	modified:   play-scalatest/src/test/scala/play/grpc/scalatest/PlayScalaTestSpec.scala
	modified:   play-specs2/src/test/scala/play/grpc/specs2/PlaySpecs2Spec.scala

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   play-interop-test-scala/src/test/scala/play/grpc/PlayScalaRouterSpec.scala

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.com