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/codegen/initializers/workers2.kt

@@ -12,6 +12,7 @@ class Z(val x: Int)
 val z = Z(42)
 
 // FILE: main.kt
+@file:OptIn(ObsoleteWorkersApi::class)
 import kotlin.native.concurrent.*
 
 fun main() {
@@ -20,4 +21,4 @@ fun main() {
     worker.execute(TransferMode.SAFE, { -> }, {
         it -> println(z.x)
     }).consume { }
-}
+}