[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

compiler/testData/builtin-classes/java6/kotlin.txt

@@ -362,7 +362,7 @@ public abstract class Enum</*0*/ E : kotlin.Enum<E>> : kotlin.Comparable<E>, jav
     /*primary*/ public constructor ExperimentalStdlibApi()
 }
 
-@kotlin.annotation.Target(allowedTargets = {AnnotationTarget.CLASS}) @kotlin.annotation.Retention(value = AnnotationRetention.BINARY) @kotlin.SinceKotlin(version = "1.8") public final annotation class ExperimentalSubclassOptIn : 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 {
     /*primary*/ public constructor ExperimentalSubclassOptIn()
 }