[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/api/js/kotlin.kt

@@ -1599,6 +1599,7 @@ public final annotation class ExperimentalStdlibApi : kotlin.Annotation {
 @kotlin.annotation.Target(allowedTargets = {AnnotationTarget.CLASS})
 @kotlin.annotation.Retention(value = AnnotationRetention.BINARY)
 @kotlin.SinceKotlin(version = "1.8")
+@kotlin.RequiresOptIn
 public final annotation class ExperimentalSubclassOptIn : kotlin.Annotation {
     public constructor ExperimentalSubclassOptIn()
 }