django/utils: Get access token renewal date from proper dict field

In the dictionary decoded from an OIDC access token, the iat field store
the token creation date while the auth_time field store the date the
OIDC session was opened.

In order to get an accurate authentication time, the date stored in
the iat field must be used as it corresponds to the time an access
token was refreshed and thus the latest valid authentication date.