Add missing @DelicateCoroutinesApi (#4116)

diff --git a/kotlinx-coroutines-core/concurrent/src/MultithreadedDispatchers.common.kt b/kotlinx-coroutines-core/concurrent/src/MultithreadedDispatchers.common.kt
index bda8d35..a05f0a6 100644
--- a/kotlinx-coroutines-core/concurrent/src/MultithreadedDispatchers.common.kt
+++ b/kotlinx-coroutines-core/concurrent/src/MultithreadedDispatchers.common.kt
@@ -59,4 +59,5 @@
  * @param name the base name of the created threads.
  */
 @ExperimentalCoroutinesApi
+@DelicateCoroutinesApi
 public expect fun newFixedThreadPoolContext(nThreads: Int, name: String): CloseableCoroutineDispatcher
diff --git a/kotlinx-coroutines-core/native/src/MultithreadedDispatchers.kt b/kotlinx-coroutines-core/native/src/MultithreadedDispatchers.kt
index 283fce0..919430c 100644
--- a/kotlinx-coroutines-core/native/src/MultithreadedDispatchers.kt
+++ b/kotlinx-coroutines-core/native/src/MultithreadedDispatchers.kt
@@ -11,6 +11,7 @@
 import kotlin.time.*
 import kotlin.time.Duration.Companion.milliseconds
 
+@DelicateCoroutinesApi
 public actual fun newFixedThreadPoolContext(nThreads: Int, name: String): CloseableCoroutineDispatcher {
     require(nThreads >= 1) { "Expected at least one thread, but got: $nThreads" }
     return MultiWorkerDispatcher(name, nThreads)