Update copyright header to include year 2021

COPYING

doc/mainpage.h

source/App/BitstreamExtractorApp/BitstreamExtractorApp.cpp

source/App/BitstreamExtractorApp/BitstreamExtractorApp.h

...ce/App/BitstreamExtractorApp/BitstreamExtractorAppCfg.cpp

source/App/BitstreamExtractorApp/BitstreamExtractorAppCfg.h

source/App/BitstreamExtractorApp/bitstreamextractormain.cpp

source/App/DecoderApp/DecApp.cpp

source/App/DecoderApp/DecApp.h

source/App/DecoderApp/DecAppCfg.cpp

source/App/DecoderApp/DecAppCfg.h

source/App/DecoderApp/decmain.cpp

source/App/EncoderApp/EncApp.cpp

source/App/EncoderApp/EncApp.h

source/App/EncoderApp/EncAppCfg.cpp

source/App/EncoderApp/EncAppCfg.h

source/App/EncoderApp/encmain.cpp

source/App/Parcat/parcat.cpp

source/App/SEIRemovalApp/SEIRemovalApp.cpp

source/App/SEIRemovalApp/SEIRemovalApp.h