SF-0033: Amend proposal in response to feedback. (swiftlang#1502)

* [Proposal] Add "String Encoding Names" proposal.

This proposal allows `String.Encoding` to be converted to and from various names.

For example:

```swift
print(String.Encoding.utf8.ianaName!) // Prints "UTF-8"
print(String.Encoding(ianaName: "ISO_646.irv:1991") == .ascii) // Prints "true"
```

* Change the link to implementation.

* Remove description about swiftlang#1015 since it is resolved.

Links:
- Issue: swiftlang#1015
- Resolvers:
  * swiftlang#1217
  * swiftlang/swift-corelibs-foundation#5194

* Add a description about already-available ICU string converter.

* SF-0033: Add `@available` attributes to sample code.

In response to: https://forums.swift.org/t/review-sf-0033-string-encoding-names/81965/7

* SF-0033: Clarify which "case-insensitivity" is used.

In response to: https://forums.swift.org/t/review-sf-0033-string-encoding-names/81965/8

* SF-0033: Change FoundationPreview version to 6.3.

In response to: swiftlang#1502 (comment)

Author: YOCKOW

Proposals/0033-String-Encoding-Names.md

@@ -159,9 +159,11 @@ We introduce a computed property that returns the name, and the initializer that
 ```swift
 extension String.Encoding {
   /// The name of this encoding that is compatible with the one of the IANA registry "charset".
+  @available(FoundationPreview 6.3, *)
   public var ianaName: String?
 
   /// Creates an instance from the name of the IANA registry "charset".
+  @available(FoundationPreview 6.3, *)
   public init?(ianaName: String)
 }
 ```
@@ -222,7 +224,7 @@ Accordingly, this proposal suggests just simple correspondence between `String.E
 
 ### Name to `String.Encoding`
 
-- `init(ianaName:)` adopts case-insensitive comparison with *Preferred MIME Name*, *Name*, and *Aliases*.
+- `init(ianaName:)` adopts ASCII case-insensitive comparison with *Preferred MIME Name*, *Name*, and *Aliases*.
 
 
 ## Source compatibility
@@ -232,7 +234,7 @@ These changes proposed here are only additive. However, care must be taken if mi
 
 ## Implications on adoption
 
-This feature can be freely adopted and un-adopted in source code with no deployment constraints and without affecting source compatibility.
+This feature can be freely adopted and un-adopted in source code without affecting source compatibility.
 
 
 ## Future directions