Migrate UseExperimental->OptIn in project sources

libraries/stdlib/test/time/TestTimeSourceTest.kt

@@ -3,7 +3,7 @@
  * 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.*
@@ -59,4 +59,4 @@ class TestTimeSourceTest {
         timeSource += 1.9.nanoseconds
         assertEquals(1.nanoseconds, mark.elapsedNow())
     }
-}
+}