Motivation for this change

https://github.com/gpg/gnupg/blob/master/NEWS

Noteworthy changes in version 2.1.21 (2017-05-15)

• gpg,gpgsm: Fix corruption of old style keyring.gpg files. This
  bug was introduced with version 2.1.20. Note that the default
  pubring.kbx format was not affected.
• gpg,dirmngr: Removed the skeleton config file support. The
  system's standard methods for providing default configuration
  files should be used instead.
• w32: The Windows installer now allows installion of GnuPG without
  Administrator permissions.
• gpg: Fixed import filter property match bug.
• scd: Removed Linux support for Cardman 4040 PCMCIA reader.
• scd: Fixed some corner case bugs in resume/suspend handling.
• Many minor bug fixes and code cleanup.

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.