Stabilize unsigned types KT-45653

Deprecate specialized unsigned iterators for removal.

Fix compiler tests:
- drop unsignedLiteralsOn1_2 because apiVersion 1.2 is no longer supported
- drop experimental unsigned literals diagnostic test

libraries/stdlib/jvm/src/kotlin/CharCodeJVM.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -10,7 +10,6 @@ package kotlin
  *
  * @sample samples.text.Chars.charFromCode
  */
-@OptIn(ExperimentalUnsignedTypes::class)
 @ExperimentalStdlibApi
 @SinceKotlin("1.4")
 @kotlin.internal.InlineOnly