[bug#73465] Wireguard: Deprecate and rename fields

[Richard Sent]

With #73955, private-key better supports g-exp based command redirection,
e.g.

--8<---------------cut here---------------start------------->8---
;; A config of
(wireguard-configuration
  ...
  (private-key (string-append "(<" my-custom-script ">")))

;; Results in
PostUp = ... set %i private-key <(/gnu/store/...-my-custom-script) ...
--8<---------------cut here---------------end--------------->8---

(This was also supported before but was more limited.)

Given that, I think renaming it to private-key-file is more confusing
than keeping it as private-key. Same for preshared-key.

Perhaps we can somehow check the field, see if the user enters a
WG-compatible key literally, and emit a warning? I'm not fluent on the
format to determine the best way for that.
-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.