How to migrate WhatsApp messages data from Android to iPhone?
From WhatsApp's FAQ, please note the emphasized note in the middle:

Changing your phone
Different types of phone
If you're moving from one type of phone to another, such as from an iPhone to an Android, and preserving your number, you'll keep your account info. This information is tied to the phone number. Simply download WhatsApp on the new phone and verify your number.
Note: You can't migrate your messages across different types of phone.
If you're moving from one type of phone to another and not preserving your number, simply download WhatsApp on the new phone and verify the new phone number.
Important: Remember to delete your old account. However, if you didn't delete your account and no longer have access to your old phone, don't worry. If the new owner of your old number activates WhatsApp on a new phone after 45 days, all of your account information tied to that phone number will be completely deleted.