gifsicle: 1.90 -> 1.91 (#36536)

Semi-automatic update. These checks were done:

- built on NixOS
- ran `/nix/store/7852k36z8l2vfsxm3ylgm0la1ggl9c8w-gifsicle-1.91/bin/gifsicle -h` got 0 exit code
- ran `/nix/store/7852k36z8l2vfsxm3ylgm0la1ggl9c8w-gifsicle-1.91/bin/gifsicle --help` got 0 exit code
- ran `/nix/store/7852k36z8l2vfsxm3ylgm0la1ggl9c8w-gifsicle-1.91/bin/gifsicle --version` and found version 1.91
- ran `/nix/store/7852k36z8l2vfsxm3ylgm0la1ggl9c8w-gifsicle-1.91/bin/gifdiff -h` got 0 exit code
- ran `/nix/store/7852k36z8l2vfsxm3ylgm0la1ggl9c8w-gifsicle-1.91/bin/gifdiff --help` got 0 exit code
- ran `/nix/store/7852k36z8l2vfsxm3ylgm0la1ggl9c8w-gifsicle-1.91/bin/gifdiff -v` and found version 1.91
- ran `/nix/store/7852k36z8l2vfsxm3ylgm0la1ggl9c8w-gifsicle-1.91/bin/gifdiff --version` and found version 1.91
- found 1.91 with grep in /nix/store/7852k36z8l2vfsxm3ylgm0la1ggl9c8w-gifsicle-1.91
- found 1.91 in filename of file in /nix/store/7852k36z8l2vfsxm3ylgm0la1ggl9c8w-gifsicle-1.91