KT-55268 Make k.c.Volatile stable

compiler/testData/codegen/box/annotations/jvmAnnotationFlags.kt

@@ -1,6 +1,5 @@
 // TARGET_BACKEND: JVM
 
-// !API_VERSION: 1.9
 // WITH_STDLIB
 // FULL_JDK
 
@@ -13,7 +12,6 @@ class CustomDelegate {
 
 class C {
     @Volatile var vol = 1
-    @OptIn(ExperimentalStdlibApi::class)
     @kotlin.concurrent.Volatile var vol2 = 1
     @Transient val tra = 1
     @delegate:Transient val del: String by CustomDelegate()