Re-enable Play/Scala tests on Java 9+ (#5593) Almost all Play/Scala tests were ignored on Java 9/10, which makes it lack coverage. This PR does:
- Update Scala version to Java9-compatible version according to [JDK compatibility](https://docs.scala-lang.org/overviews/jdk-compatibility/overview.html). - Update default Play version to latest 2.6.15, which supports Java 9/10 - Adjust corresponding samples. Previous default Play version 2.3.10 was too old (release at 2014). - Ignored the tests which can't work on Java 9+. - Separate Play test resources with different versions. For example, Play 2.6 uses `build.gradle` and `Play 2.4/2.5` uses `build.gradle.old`. - Previously `AbstractMultiVersionPlayContinuousBuildIntegrationTest` didn't have `targetPlatform` configuration, which made multi-version tests meaningless - Fix `play-logback` configuration for `2.5.18`.