[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/builtIns/kotlin.txt

@@ -601,7 +601,7 @@ public abstract class Enum<E : R|kotlin/Enum<E>|> : R|kotlin/Comparable<E>|, R|j
 
 }
 
-@R|kotlin/annotation/Target|(allowedTargets = <implicitArrayOf>(R|kotlin/annotation/AnnotationTarget.CLASS|)) @R|kotlin/annotation/Retention|(value = R|kotlin/annotation/AnnotationRetention.BINARY|) @R|kotlin/SinceKotlin|(version = String(1.8)) public final annotation class ExperimentalSubclassOptIn : R|kotlin/Annotation| {
+@R|kotlin/annotation/Target|(allowedTargets = <implicitArrayOf>(R|kotlin/annotation/AnnotationTarget.CLASS|)) @R|kotlin/annotation/Retention|(value = R|kotlin/annotation/AnnotationRetention.BINARY|) @R|kotlin/SinceKotlin|(version = String(1.8)) @R|kotlin/RequiresOptIn|() public final annotation class ExperimentalSubclassOptIn : R|kotlin/Annotation| {
     public constructor(): R|kotlin/ExperimentalSubclassOptIn|
 
 }

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
 

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java

@@ -39943,6 +39943,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
                     runTest("compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/CorrectApplicability.kt");
                 }
 
+                @Test
+                @TestMetadata("Experimentality.kt")
+                public void testExperimentality() throws Exception {
+                    runTest("compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/Experimentality.kt");
+                }
+
                 @Test
                 @TestMetadata("IncorrectApplicability.kt")
                 public void testIncorrectApplicability() throws Exception {

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java

@@ -40063,6 +40063,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
                     runTest("compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/CorrectApplicability.kt");
                 }
 
+                @Test
+                @TestMetadata("Experimentality.kt")
+                public void testExperimentality() throws Exception {
+                    runTest("compiler/testData/diagnostics/testsWithStdLib/annotations/subclassOptInRequired/Experimentality.kt");
+                }
+
                 @Test
                 @TestMetadata("IncorrectApplicability.kt")
                 public void testIncorrectApplicability() throws Exception {