Fix minimum Swift version for FirebaseFirestoreSwift (#6248)

`FirebaseFirestoreSwift` was already using synthesized `Equatable` and `Hashable` conformance which requires Swift 4.1.

FirebaseFirestoreSwift.podspec

@@ -21,7 +21,7 @@ Google Cloud Firestore is a NoSQL document database built for automatic scaling,
     :tag => 'FirestoreSwift-' + s.version.to_s
   }
 
-  s.swift_version           = '4.0'
+  s.swift_version           = '4.1'
   s.ios.deployment_target   = '8.0'
   s.osx.deployment_target   = '10.11'
   s.tvos.deployment_target  = '10.0'

Firestore/Swift/CHANGELOG.md

@@ -1,6 +1,9 @@
 # Unreleased
 - Removed support for wrapping `NSDate` in a `@ServerTimestamp` property
   wrapper. This never actually worked because `NSDate` is not `Codable`.
+- Fixed the minimum supported Swift version to be 4.1. This was already
+  effectively the case because the code made use of Swift 4.1 features without
+  documenting this requirement.
 
 # v0.3
 - Renamed the misspelled `FirestoreDecodingError.fieldNameConfict` to