update copyright date from `2020` to `2021`
Please update copyright date from 2020
to 2021
https://git.jami.net/savoirfairelinux/ring-project/issues/543
https://git.jami.net/savoirfairelinux/ring-client-gnome/issues/1127
https://git.jami.net/savoirfairelinux/ring-client-android/issues/752
Thank you