poppler: update to version 0.56.0

Previous versions have a ton of bugs that may be used in DoS attacks, i.e. by
making poppler-based services crash when processing specially crafted PDF
documents.

pkgs/development/libraries/poppler/default.nix

···
 }:
 
 let # beware: updates often break cups-filters build
-  version = "0.50.0";
-  sha256 = "0dmwnh59m75vhii6dw63x8l0qa0ha733pb8bdqzr7lw9nwc37jf9";
+  version = "0.56.0";
+  sha256 = "0wviayidfv2ix2ql0d4nl9r1ia6qi5kc1nybd9vjx27dk7gvm7c6";
 in
 stdenv.mkDerivation rec {
   name = "poppler-${suffix}-${version}";