Deprecate Char-to-Number conversions in stdlib (JVM and JS)

- Int.toChar was left non-deprecated because the replacement is not intrinsic yet.
- Number.toChar was left non-deprecated because otherwise the deprecation propagates to the override, Int.toChar.

KT-23451

libraries/stdlib/test/text/StringEncodingTest.kt

@@ -47,7 +47,7 @@ class StringEncodingTest {
 
     // https://youtrack.jetbrains.com/issue/KT-31614
     private fun string(vararg codeUnits: Int): String {
-        return buildString { codeUnits.forEach { append(it.toChar()) } }
+        return buildString { codeUnits.forEach { append(Char(it)) } }
     }
 
     @Test