Remove Swift language comment from README

I think it confuses rather than clarifies. It might make users think
that they need to be using Swift 6 in _their_ project, which they don’t.

README.md

@@ -13,7 +13,7 @@ This is the repository for the Swift version of the Ably Chat SDK. We aim to bui
 
 ## Requirements
 
-Xcode 16 (i.e. a compiler that supports Swift 6) or later.
+Xcode 16 or later.
 
 ## Installation