概要

Unsupported Target iOS SDK

Target SDK selected in the iOS Player Settings (120) 
is not supported by the Cocoapods included in this project. 
The build will very likely fail. 
The minimum supported version is "13.0" required by pods 
(Firebase/Analytics, Firebase/Auth, Firebase/Core, Firebase/Crashlytics, Firebase/Firestore, Firebase/Messaging, Firebase/RemoteConfig)
Would you like to update the target SDK version?

Unity プロジェクトに導入している Firebase を
11.8.1 から 12.5.0 にアップデートしたら
上記のダイアログが表示される現象に遭遇した。