[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/fir/analysis-tests/testData/resolveWithStdlib/ClassIdDiagnosticRendering.fir.txt

@@ -1,4 +1,5 @@
 FILE: ClassIdDiagnosticRendering.kt
+    @FILE:R|kotlin/OptIn|(markerClass = vararg(<getClass>(Q|kotlin/ExperimentalSubclassOptIn|)))
     package a.b.c
 
     @R|kotlin/RequiresOptIn|() public final annotation class Marker : R|kotlin/Annotation| {

compiler/fir/analysis-tests/testData/resolveWithStdlib/ClassIdDiagnosticRendering.kt

@@ -1,4 +1,5 @@
 // WITH_EXTENDED_CHECKERS
+@file:OptIn(ExperimentalSubclassOptIn::class)
 
 package a.b.c