imagemagick6: 6.9.12-19 -> 6.9.12-26

[erictapen]

From Changelog:

2021-07-26  6.9.12-20  <quetzlzacatenango@image...>
  * Heap-based buffer overflow in TIFF coder (alert from Hunter Mitchell).
  * Grayscale image write optimization

Motivation for this change

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[risicle]

Successfully built imagemagick6_light imagemagick6 and imagemagick6Big on macos 10.15 & nixos x86_64

[github-actions]

Successfully created backport PR #142944 for release-21.05.