editors/libreoffice6: update to 6.4.7 release

LibreOffice 6.4.7 is now available – it's the final release of the 6.4 family,
and includes over 70 bugfixes and compatibility improvements:

https://blog.documentfoundation.org/blog/2020/10/22/tdf-announces-libreoffice-647/

MFH: 2020Q4 (hat: ports-secteam)