Migrate UseExperimental->OptIn in project sources

compiler/testData/codegen/box/inference/kt35684.kt

@@ -13,7 +13,7 @@ fun test() {
     }
 }
 
-@UseExperimental(ExperimentalTypeInference::class)
+@OptIn(ExperimentalTypeInference::class)
 fun <U> sequence(@BuilderInference block: suspend Inv<U>.() -> Unit) {}
 
 interface Inv<T> {
@@ -22,4 +22,4 @@ interface Inv<T> {
 
 fun <K> materialize(): Inv<K> = TODO()
 
-fun box(): String = "OK"
+fun box(): String = "OK"