Migrate UseExperimental->OptIn in project sources

libraries/stdlib/test/time/TimeMarkTest.kt

@@ -2,7 +2,8 @@
  * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
-@file:UseExperimental(ExperimentalTime::class)
+
+@file:OptIn(ExperimentalTime::class)
 package test.time
 
 import kotlin.test.*
@@ -50,4 +51,4 @@ class TimeMarkTest {
         markPast1.assertHasPassed(true)
 
     }
-}
+}