[Experiment] default to Loom on

[yschimke]

For discussion of #8167

Unlikely to land

[JakeWharton]

Seems like a better case for MR jars than a runtime check. This is going to cause deoptimization on Android and errors in ProGuard/R8/Graal nativeimage as written.

[yschimke]

No plan to land, closing.

Failing on API checks

> Task :okhttp:animalsnifferMain FAILED

[Undefined reference | android-api-level-21-5.0.1_r2] okhttp3.(Dispatcher.kt:105)
  >> java.util.concurrent.ExecutorService java.util.concurrent.Executors.newVirtualThreadPerTaskExecutor()

[Undefined reference | java18-1.0] okhttp3.(Dispatcher.kt:105)
  >> java.util.concurrent.ExecutorService java.util.concurrent.Executors.newVirtualThreadPerTaskExecutor()