remove platform version check from Messaging header (#8492)

Author: morganchen12

FirebaseMessaging/CHANGELOG.md

@@ -1,4 +1,5 @@
-# 2021-07 -- v8.5.0
+# 2021-08 -- v8.6.0
+- [changed] Removed iOS version check from `FIRMessagingExtensionHelper.h` (#8492).
 - [added] Added new API `FIRMessagingExtensionHelper exportDeliveryMetricsToBigQuery` that allows developers to enable notification delivery metrics that exports to BigQuery. (#6181)
 
 # 2021-06 -- v8.2.0

FirebaseMessaging/Sources/Public/FirebaseMessaging/FIRMessagingExtensionHelper.h

@@ -14,8 +14,11 @@
  * limitations under the License.
  */
 
-#if __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_10_0 || \
-    __MAC_OS_X_VERSION_MAX_ALLOWED >= __MAC_10_14
+#import <Foundation/Foundation.h>
+
+@class UNMutableNotificationContent, UNNotificationContent;
+
+#if __has_include(<UserNotifications/UserNotifications.h>)
 #import <UserNotifications/UserNotifications.h>
 #endif