Stabilize JsonElement DSL functions with Nothing? overload

They help avoiding writing `JsonPrimitive(null as String?)`

Author: sandwwraith

formats/json/commonMain/src/kotlinx/serialization/json/JsonElement.kt

@@ -101,7 +101,6 @@ public fun JsonPrimitive(value: String?): JsonPrimitive {
 }
 
 /** Creates [JsonNull]. */
-@ExperimentalSerializationApi
 @Suppress("FunctionName", "UNUSED_PARAMETER") // allows to call `JsonPrimitive(null)`
 public fun JsonPrimitive(value: Nothing?): JsonNull = JsonNull
 

formats/json/commonMain/src/kotlinx/serialization/json/JsonElementBuilders.kt

@@ -122,7 +122,6 @@ public fun JsonObjectBuilder.put(key: String, value: String?): JsonElement? = pu
  * Returns the previous value associated with [key], or `null` if the key was not present.
  */
 @IgnorableReturnValue
-@ExperimentalSerializationApi
 @Suppress("UNUSED_PARAMETER") // allows to call `put("key", null)`
 public fun JsonObjectBuilder.put(key: String, value: Nothing?): JsonElement? = put(key, JsonNull)
 
@@ -189,7 +188,6 @@ public fun JsonArrayBuilder.add(value: String?): Boolean = add(JsonPrimitive(val
  * Always returns `true` similarly to [ArrayList] specification.
  */
 @IgnorableReturnValue
-@ExperimentalSerializationApi
 @Suppress("UNUSED_PARAMETER") // allows to call `add(null)`
 public fun JsonArrayBuilder.add(value: Nothing?): Boolean = add(JsonNull)