v8.10.6

Release 8.10.6

• Update dependencies to latest where possible.
• Switch from zlib to zlib-ng (lovell/sharp-libvips#25).
• Build a more minimal libxml2 (lovell/sharp-libvips#92).
• Include libimagequant as dependency (lovell/sharp-libvips#91).
• Allow linker to remove unused sections (lovell/sharp-libvips#88).
• Build aom without HBR/WebM support (#23).
• Switch from libjpeg-turbo to MozJPEG (#21).
• Binaries are being built with LLVM 11.1 (was GCC 10.2).
• Remove broken HDF5 dependency in the -all variant.