Update the date (2021) in the headers
As we are starting a new year, we need to update the headers with the good date for the copyright.
We should wait until OTB 8.0 is ready, so this action doesn't interfere with the ongoing developments.
The command to perform the update:
git grep -lE "Copyright \(C\) (20..-)?20.. Centre National d'Etudes Spatiales \(CNES\)" | xargs sed -i -e 's/\(Copyright (C) \)\(20..-\)\?20..\( Centre National .*\)/\1\22021\3/'