Declaring `services.dovecot2.mailboxes' as a list is deprecated and will break eval in 21.03!

Issue description

After an update, nixos reports this warning:

trace: warning: Declaring `services.dovecot2.mailboxes' as a list is deprecated and will break eval in 21.03!

Due to https://github.com/NixOS/nixpkgs/pull/89486

Technical details

 - system: `"x86_64-linux"`
 - host os: `Linux 5.7.8, NixOS, 20.09pre234194.8d05772134f (Nightingale)`
 - multi-user?: `no`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.7`
 - channels(root): `"home-manager, nixos-20.09pre234194.8d05772134f"`
 - channels(josephtheengineer): `"home-manager"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

SNM Version

master