[k1/K2]: Mark @SubclassOptInRequired as an experimental

At the moment, SubclassOptInRequired is marked with the
ExperimentalSubclassOptIn annotation. However, it does not work
as expected due to a missing opt-in error. To use SubclassOptInRequired,
an explicit opt-in is necessary because SubclassOptInRequired is an
unstable feature now.

^KT-64739

libraries/stdlib/src/kotlin/annotations/OptIn.kt

@@ -144,6 +144,7 @@ public annotation class OptIn(
 @Target(CLASS)
 @Retention(BINARY)
 @SinceKotlin("1.8")
+@RequiresOptIn
 public annotation class ExperimentalSubclassOptIn
 
 /**