You would require to migrate to Android X if you intend to use features that have breaking changes for the latest library. I personally faced this problem cloud_firestore 0.9.0 as the change log says, it required a migration to Android X.
Your Android app is in the android directory of the project. First open the android directory with Android Studio 3.2+. Then Refactor > Migrate to AndroidX. After that also add these to the gradle.properties file in the android directory
android.useAndroidX=true
android.enableJetifier=true
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