Bump copyright year to 2021

- Substitute occurrences of "-2020" with "-2021" using:
  git grep -I -l -e '-2020' -- ':(exclude)*.bak' | xargs \
  sed -b -i -e 's/-2020/-2021/g'

CUETools.Codecs.FLACCL/FLACCLWriter.cs

@@ -1,6 +1,6 @@
 /**
  * CUETools.FLACCL: FLAC audio encoder using CUDA
- * Copyright (c) 2009-2020 Grigory Chudov
+ * Copyright (c) 2009-2021 Grigory Chudov
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public