Add available(iOS 15.0, macOS 12.0, ..., *) annotation

Author: andrewheard

FirebaseAI/Tests/Unit/Types/Generable/GenerableTests.swift

@@ -15,6 +15,7 @@
 @testable import FirebaseAILogic
 import XCTest
 
+@available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
 final class GenerableTests: XCTestCase {
   func testInitializeGenerableTypeFromModelOutput() throws {
     let addressProperties: [(String, any ConvertibleToModelOutput)] =