[SPM] Require Xcode 13.3+ (Swift 5.6) (#11046)

Increased swift-tools-version to 5.6 in Package.swift to declare that that the Swift 5.6 toolchain (included in Xcode 13.3) is the minimum version required to build the Firebase package.

Replaced uses of initializers / methods / enums that were deprecated in SPM 5.6.

Note: Xcode 13.3.1+ has been the minimum supported since Firebase 9.0 but this enforces Xcode 13.3+ in Package.swift. However, this does not enforce Xcode 13.3.1 specifically since the Swift toolchain version did not change between Xcode 13.3 and 13.3.1.

Author: andrewheard

FirebaseCore/CHANGELOG.md

@@ -1,5 +1,6 @@
 # Firebase 10.8.0
 - Fix new build warnings introduced by Xcode 14.3. (#11059)
+- [changed] The Firebase Swift package now requires the Swift 5.6 toolchain (Xcode 13.3) to build.
 
 # Firebase 10.4.0
 - Deprecate `androidClientID` and `trackingID` from FirebaseOptions. (#10520)