Allow building on jdk19

[raboof]

Building on jdk19 currently fails because of a newly added deprecation in sun.misc.Unsafe:

[error] actor/src/main/scala/org/apache/pekko/actor/LightArrayRevolverScheduler.scala:350:41: method objectFieldOffset in class Unsafe is deprecated
[error]   private[this] val taskOffset = unsafe.objectFieldOffset(classOf[TaskHolder].getDeclaredField("task"))

Perhaps we can exclude this warning (until #417 is in place), though for some reason -Wconf:msg=method objectFieldOffset in class Unsafe is deprecated:silent didn't seem to be sufficient.

[He-Pin]

Maybe just need to add a @nowarn("msg=deprecated")?

[raboof]

Nice!

[pjfanning]

So, after #467 it appears you can compile and test Pekko with JDK 20.

Paradox plugin - for building reference docs - fails with

[error] java.lang.RuntimeException: Error creating extended parser class: Could not determine whether class 'org.pegdown.ParserWithDirectives$$parboiled' has already been loaded
[error] 	at org.parboiled.Parboiled.createParser(Parboiled.java:58)
[error] 	at com.lightbend.paradox.markdown.Reader.<init>(Reader.scala:40)
[error] 	at com.lightbend.paradox.sbt.ParadoxPlugin$.$anonfun$baseParadoxSettings$2(ParadoxPlugin.scala:83)
[error] 	at scala.Function1.$anonfun$compose$1(Function1.scala:49)
[error] 	at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:62)
[error] 	at sbt.std.Transform$$anon$4.work(Transform.scala:68)
[error] 	at sbt.Execute.$anonfun$submit$2(Execute.scala:282)
[error] 	at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:23)
[error] 	at sbt.Execute.work(Execute.scala:291)
[error] 	at sbt.Execute.$anonfun$submit$1(Execute.scala:282)
[error] 	at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
[error] 	at sbt.CompletionService$$anon$2.call(CompletionService.scala:64)
[error] 	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317)
[error] 	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:577)
[error] 	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317)
[error] 	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
[error] 	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
[error] 	at java.base/java.lang.Thread.run(Thread.java:1623)
[error] Caused by: java.lang.RuntimeException: Could not determine whether class 'org.pegdown.ParserWithDirectives$$parboiled' has already been loaded
[error] 	at org.parboiled.transform.AsmUtils.findLoadedClass(AsmUtils.java:217)
[error] 	at org.parboiled.transform.ParserTransformer.transformParser(ParserTransformer.java:35)
[error] 	at org.parboiled.Parboiled.createParser(Parboiled.java:54)
[error] 	at com.lightbend.paradox.markdown.Reader.<init>(Reader.scala:40)
[error] 	at com.lightbend.paradox.sbt.ParadoxPlugin$.$anonfun$baseParadoxSettings$2(ParadoxPlugin.scala:83)
[error] 	at scala.Function1.$anonfun$compose$1(Function1.scala:49)
[error] 	at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:62)
[error] 	at sbt.std.Transform$$anon$4.work(Transform.scala:68)
[error] 	at sbt.Execute.$anonfun$submit$2(Execute.scala:282)
[error] 	at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:23)
[error] 	at sbt.Execute.work(Execute.scala:291)
[error] 	at sbt.Execute.$anonfun$submit$1(Execute.scala:282)
[error] 	at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
[error] 	at sbt.CompletionService$$anon$2.call(CompletionService.scala:64)
[error] 	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317)
[error] 	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:577)
[error] 	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317)
[error] 	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
[error] 	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
[error] 	at java.base/java.lang.Thread.run(Thread.java:1623)
[error] Caused by: java.lang.reflect.InaccessibleObjectException: Unable to make protected final java.lang.Class java.lang.ClassLoader.findLoadedClass(java.lang.String) accessible: module java.base does not "opens java.lang" to unnamed module @6ae027fd
[error] 	at java.base/java.lang.reflect.AccessibleObject.throwInaccessibleObjectException(AccessibleObject.java:387)
[error] 	at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:363)
[error] 	at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:311)
[error] 	at java.base/java.lang.reflect.Method.checkCanSetAccessible(Method.java:201)
[error] 	at java.base/java.lang.reflect.Method.setAccessible(Method.java:195)
[error] 	at org.parboiled.transform.AsmUtils.findLoadedClass(AsmUtils.java:210)
[error] 	at org.parboiled.transform.ParserTransformer.transformParser(ParserTransformer.java:35)
[error] 	at org.parboiled.Parboiled.createParser(Parboiled.java:54)
[error] 	at com.lightbend.paradox.markdown.Reader.<init>(Reader.scala:40)