[Core] Fix iOS 12 spec lint failure from incomplete FirebaseCoreInternal implementation

Firebase.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name             = 'Firebase'
-  s.version          = '11.4.1'
+  s.version          = '11.4.2'
   s.summary          = 'Firebase'
 
   s.description      = <<-DESC
@@ -43,7 +43,7 @@ Simplify your app development, grow your user base, and monetize more effectivel
   end
 
   s.subspec 'CoreOnly' do |ss|
-    ss.dependency 'FirebaseCore', '11.4.1'
+    ss.dependency 'FirebaseCore', '11.4.2'
     ss.source_files = 'CoreOnly/Sources/Firebase.h'
     ss.preserve_paths = 'CoreOnly/Sources/module.modulemap'
     if ENV['FIREBASE_POD_REPO_FOR_DEV_POD'] then

FirebaseCore.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name             = 'FirebaseCore'
-  s.version          = '11.4.1'
+  s.version          = '11.4.2'
   s.summary          = 'Firebase Core'
 
   s.description      = <<-DESC

FirebaseCore/CHANGELOG.md

@@ -1,3 +1,7 @@
+# Firebase 11.4.2
+- [fixed] CocoaPods only release to fix iOS 12 build failure resulting from
+  missing OS availability check in the `FirebaseCoreExtension` SDK.
+
 # Firebase 11.4.1
 - [fixed] CocoaPods only release to revert breaking change in
   `FirebaseCoreExtension` SDK. (#13942)

FirebaseCore/Internal/Sources/HeartbeatLogging/_ObjC_HeartbeatController.swift

@@ -49,10 +49,15 @@ public class _ObjC_HeartbeatController: NSObject {
   ///
   /// - Note: This API is thread-safe.
   /// - Returns: A heartbeats payload for the flushed heartbeat(s).
-  @available(iOS 13.0, macOS 10.15, macCatalyst 13.0, tvOS 13.0, watchOS 6.0, *)
-  public func flushAsync() async -> _ObjC_HeartbeatsPayload {
-    let heartbeatsPayload = await heartbeatController.flushAsync()
-    return _ObjC_HeartbeatsPayload(heartbeatsPayload)
+  public func flushAsync(completionHandler: @escaping (_ObjC_HeartbeatsPayload) -> Void) {
+    if #available(iOS 13.0, macOS 10.15, macCatalyst 13.0, tvOS 13.0, watchOS 6.0, *) {
+      Task {
+        let heartbeatsPayload = await heartbeatController.flushAsync()
+        completionHandler(_ObjC_HeartbeatsPayload(heartbeatsPayload))
+      }
+    } else {
+      completionHandler(_ObjC_HeartbeatsPayload(HeartbeatsPayload.emptyPayload))
+    }
   }
 
   /// Synchronously flushes the heartbeat for today.

FirebaseCore/Sources/FIRHeartbeatLogger.m

@@ -87,7 +87,8 @@ - (FIRHeartbeatsPayload *)flushHeartbeatsIntoPayload {
 }
 
 - (void)flushHeartbeatsIntoPayloadWithCompletionHandler:
-    (void (^)(FIRHeartbeatsPayload *))completionHandler {
+    (void (^)(FIRHeartbeatsPayload *))completionHandler
+    API_AVAILABLE(ios(13.0), macosx(10.15), macCatalyst(13.0), tvos(13.0), watchos(6.0)) {
   [_heartbeatController flushAsyncWithCompletionHandler:^(FIRHeartbeatsPayload *payload) {
     completionHandler(payload);
   }];