Add list of removals for tracking purposes

Signed-off-by: Alex Saveau <saveau.alexandre@gmail.com>

Author: SUPERCILEX

auth/upgrade-guide.md

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+# Removals
+
+- `AuthUI#signOut(Activity)` and `AuthUI#delete(Activity)` -> `AuthUI#signOut(AppCompatActivity)` and `AuthUI#delete(AppCompatActivity)`
+- `AuthUI.SignInIntentBuilder#setProviders(String...)` -> `AuthUI.SignInIntentBuilder#setProviders(List<IdpConfig>)`
+- `com.firebase.ui.auth.ui.ResultCodes` -> Check for `Activity#RESULT_CANCELLED` and use `IdpResponse#getErrorCode()`