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/memory/stable_ref_cross_thread_check.kt

@@ -2,7 +2,7 @@
  * Copyright 2010-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
  * that can be found in the LICENSE file.
  */
-@file:OptIn(FreezingIsDeprecated::class)
+@file:OptIn(FreezingIsDeprecated::class, ObsoleteWorkersApi::class)
 
 package runtime.memory.stable_ref_cross_thread_check