graphics/darktable38: update to 3.8.1

https://github.com/darktable-org/darktable/releases/tag/release-3.8.1

Rename option from LIBAVIF to AVIF.

Convert all <opt>_CMAKE_OFF to <opt>_CMAKE_BOOL for better control
of options.

After receiving dumbbell's feedback that 3.8 can be merged back
to graphics/darktable, deprecate this port for end of March.