nixos/rspamd: Allow worker type to be proxy again

When reworking the rspamd workers I disallowed `proxy` as a type and
instead used `rspamd_proxy` which is the correct name for that worker
type. That change breaks peoples existing config and so I have made this
commit which allows `proxy` as a worker type again but makes it behave
as `rspamd_proxy` and prints a warning if you use it.