opendkim: Create 2048 bit keys be default

[ismell]

The opendkim package will create a default key if one is not found:
https://github.com/NixOS/nixpkgs/blob/nixos-24.05/nixos/modules/services/mail/opendkim.nix#L136

We should pass in the -b 2028 argument to generate more secure keys by default.

See https://sendgrid.com/en-us/blog/2048-bit-dkim-keys for the recommendation.

We could also make the key length configurable if we thought it was necessary.