Historically we used apt-key to import out repository signing key into the system keyring. However, the apt-key utility is already deprecated in Bookworm and will be completely removed in Trixie, so we should switch to referencing the key in the repo file instead.

The change can be imported from https://github.com/vyos/vyos-build/pull/978/commits/2c565ad689691c336e98d57e2fa33c11578b45db