Update cups-filters to 1.28.6

NEWS - OpenPrinting CUPS Filters v1.28.6 - 2020-12-02
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CHANGES IN V1.28.6

- libcupsfilters: In generated PPDs add a grayscale mode if
  there are only color printing modes (from OpenPrinting
  CUPS).
- libcupsfilters: In generated PPDs add an "OutputBin" option
  also if it has only one choice (OpenPrinting CUPS pull
  request #18).
- libcupsfilters: Generated PPDs could have an "Unknown"
  default InputSlot (OpenPrinting CUPS issue #44).
- cups-browsed: Removed unneeded IPP attribute additions
  preventing the created local queues from preserving a
  location or description the user assigns to them (Issue
  #323).
- cups-browsed: Removed all calls of the resolve_uri() function
  of libcupsfilters, as these are not actually needed and in case
  the supplied DNS-SD-based URI is not resolvable, the function
  gets stuck for ~5 seconds.
- cups-browsed: Fixed several memory leaks, mainly from the
  code to merge printer IPP attributes for clusters (Pull
  request #322).
- cups-browsed: Silenced compiler warning.
- foomatic-rip: Fix infinite loop and input from file on raw
  printing (Pull request #318).
- foomatic-rip: Remove temporary file created during pdf-to-ps
  conversion (Pull request #313).

(prlw1)