Add available(iOS 15.0, macOS 12.0, ..., *) to GenerableTests

Author: andrewheard

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

@@ -15,6 +15,7 @@
 @testable import FirebaseAILogic
 import Testing
 
+@available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
 struct GenerableTests {
   @Test
   func initializeGenerableTypeFromModelOutput() throws {