v2.21.0

This is a major Buildroot and Linux update. If you use circuits_gpio, it is
critical to update to 1.2.0 or later to get a workaround for GPIO indices
changing.

Please see the nerves_system_br v1.26.0 release notes
for upgrade instructions if you've forked this system.

• Updated dependencies
  • linux 6.1.69-ti-r21
  • Erlang/OTP 26.2.2
  • nerves_system_br v1.26.1
  • Buildroot 2023.11.1