[Docs] Update min. Xcode version in README.md and SwiftPackageManager.md (#14918)

Author: ncooke3

README.md

@@ -86,7 +86,7 @@ For details on using Firebase from a Framework or a library, refer to [firebase_
 To develop Firebase software in this repository, ensure that you have at least
 the following software:
 
-* Xcode 15.2 (or later)
+* Xcode 16.2 (or later)
 
 CocoaPods is still the canonical way to develop, but much of the repo now supports
 development with Swift Package Manager.

SwiftPackageManager.md

@@ -2,7 +2,7 @@
 
 ## Requirements
 
-- Requires Xcode 15.2 or above
+- Requires Xcode 16.2 or above
 - Analytics requires clients to add `-ObjC` linker option.
 - See [Package.swift](Package.swift) for supported platform versions.