Migrate UseExperimental->OptIn in project sources

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/printer/builder.kt

@@ -64,7 +64,7 @@ private fun SmartPrinter.printBuilder(builder: Builder) {
             is IntermediateBuilder -> builder.materializedElement!!.typeWithArguments.replace(Regex("<.>"), "<*>")
         }
         if (builder is LeafBuilder && builder.implementation.isPublic) {
-            println("@UseExperimental(FirImplementationDetail::class)")
+            println("@OptIn(FirImplementationDetail::class)")
         }
         if (builder.parents.isNotEmpty()) {
             print("override ")
@@ -226,10 +226,10 @@ private fun SmartPrinter.printDslBuildFunction(
 ) {
     val isEmpty = builder.allFields.isEmpty()
     if (!isEmpty) {
-        println("@UseExperimental(ExperimentalContracts::class)")
+        println("@OptIn(ExperimentalContracts::class)")
         print("inline ")
     } else if(builder.implementation.isPublic) {
-        println("@UseExperimental(FirImplementationDetail::class)")
+        println("@OptIn(FirImplementationDetail::class)")
     }
     print("fun ")
     builder.implementation.element.typeArguments.takeIf { it.isNotEmpty() }?.let {

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/printer/implementation.kt

@@ -46,7 +46,7 @@ fun SmartPrinter.printImplementation(implementation: Implementation) {
 
     with(implementation) {
         if (requiresOptIn) {
-            println("@UseExperimental(FirImplementationDetail::class)")
+            println("@OptIn(FirImplementationDetail::class)")
         }
         if (!isPublic) {
             print("internal ")