Remove @ExperimentalSubclassOptIn from SubclassOptInRequired

In Kotlin 2.1, SubclassOptInRequired is positioned as stable. The
ExperimentalSubclassOptIn annotation should be removed from
SubclassOptInRequired before the 2.1 release.

^KT-71222 Fixed

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

@@ -172,10 +172,11 @@ public annotation class ExperimentalSubclassOptIn
  * @property markerClass specifies marker annotation that require explicit opt-in.
  * @see RequiresOptIn for a detailed description of opt-in semantics and propagation rules.
  */
+@Suppress("NEWER_VERSION_IN_SINCE_KOTLIN")
 @Target(CLASS)
 @Retention(BINARY)
-@SinceKotlin("1.8")
-@ExperimentalSubclassOptIn
+@SinceKotlin("2.1")
+@WasExperimental(ExperimentalSubclassOptIn::class)
 public annotation class SubclassOptInRequired(
     val markerClass: KClass<out Annotation>
 )