Make MutableMap.set return Unit

#KT-7853 Fixed

libraries/stdlib/src/kotlin/collections/MapsJVM.kt

@@ -14,7 +14,13 @@ import java.util.concurrent.ConcurrentMap
  * Allows to use the index operator for storing values in a mutable map.
  */
 // this code is JVM-specific, because JS has native set function
-public operator fun <K, V> MutableMap<K, V>.set(key: K, value: V): V? = put(key, value)
+public operator fun <K, V> MutableMap<K, V>.set(key: K, value: V): Unit {
+    put(key, value)
+}
+
+@Deprecated("Provided for binary compatibility", level = DeprecationLevel.HIDDEN)
+@JvmName("set")
+public fun <K, V> MutableMap<K, V>.set(key: K, value: V): V? = put(key, value)
 
 /**
  * getOrPut is not supported on [ConcurrentMap] since it cannot be implemented correctly in terms of concurrency.