[Vertex AI] Add Swift major version to language tag (#12829)

Author: andrewheard

FirebaseVertexAI/Sources/GenerativeAIService.swift

@@ -18,8 +18,8 @@ import Foundation
 
 @available(iOS 15.0, macOS 11.0, macCatalyst 15.0, *)
 struct GenerativeAIService {
-  /// The language of the SDK in the format "gl-<language>/<version>" where version may be blank.
-  static let languageTag = "gl-swift/"
+  /// The language of the SDK in the format "gl-<language>/<version>".
+  static let languageTag = "gl-swift/5"
 
   /// The Firebase SDK version in the format "fire/<version>".
   static let firebaseVersionTag = "fire/\(FirebaseVersion())"