Revert "Promote String <-> utf8 and CharArray conversions to stable"

This reverts commit aba97033

libraries/stdlib/src/kotlin/text/CharacterCodingException.kt

@@ -8,6 +8,6 @@ package kotlin.text
 /**
  *  The exception thrown when a character encoding or decoding error occurs.
  */
-@SinceKotlin("1.4")
-@WasExperimental(ExperimentalStdlibApi::class)
+@SinceKotlin("1.3")
+@ExperimentalStdlibApi
 public expect open class CharacterCodingException() : Exception