Mark all Worker-related API as obsolete.

* Explain what obsolete means and why workers are obsolete
* Add some documentation to workers to explain its common pitfalls

^KT-54702


Merge-request: KT-MR-9563
Merged-by: Vsevolod Tolstopyatov <qwwdfsad@gmail.com>

kotlin-native/backend.native/tests/framework/multiple/shared/shared.kt

@@ -1,4 +1,5 @@
-import kotlin.native.concurrent.Worker
+@file:OptIn(ObsoleteWorkersApi::class)
+import kotlin.native.concurrent.*
 
 object RuntimeState {
     fun produceChange() {