Postpone introducing common StringBuilder.capacity() function

Its support complicates JS StringBuilder implementation with no actual
performance improvements. Benefits of having capacity() function
in common code are also not completely clear.

Relates to KT-33069
#KT-40168

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

@@ -90,8 +90,9 @@ expect class StringBuilder : Appendable, CharSequence {
      *
      * The capacity is the maximum length this string builder can have before an allocation occurs.
      */
-    @SinceKotlin("1.4")
-    @WasExperimental(ExperimentalStdlibApi::class)
+    @SinceKotlin("1.3")
+//    @ExperimentalStdlibApi
+    @Deprecated("Obtaining StringBuilder capacity is not supported in JS and common code.", level = DeprecationLevel.ERROR)
     fun capacity(): Int
 
     /**