fix(ios): cleaning firebase-messaging-core iOS build warnings

[gabrielbiga]

UNNotificationPresentationOptions.alert is deprecated since iOS 14.0. This commit aims to fix this following the official documentation.

Deprecated Use list and banner instead.

https://developer.apple.com/documentation/usernotifications/unnotificationpresentationoptions/alert

[triniwiz]

Alert should still be used for older APIs not removed

[gabrielbiga]

Hello @triniwiz, good catch. Please take a look again, now its taking care of old platforms and still get rid of the warning.

[triniwiz]

Perfect 👌🏽