v0.21.0

This is a Buildroot version update that appears to mostly contain bug and
security fixes. It should be a low risk upgrade from v0.20.2.

• Updated dependencies
  • nerves_system_br v1.24.0
  • Buildroot 2023.05.2, 2023.05.1, 2023.05
  • Erlang/OTP 26.1