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/runtime/basic/worker_random.kt

@@ -3,6 +3,7 @@
  * that can be found in the LICENSE file.
  */
 
+@file:OptIn(ObsoleteWorkersApi::class)
 package runtime.basic.worker_random
 
 import kotlin.native.concurrent.*
@@ -39,4 +40,4 @@ fun testRandomWorkers() {
     workers.forEach {
         it.requestTermination().result
     }
-}
+}