Advisory deprecation notices surfaced by the installer tasks.

mix phoenix_kit.install and mix phoenix_kit.update print these so a host learns about upcoming changes while running a task they already run. Nothing here changes host code or behaviour — the deprecated features keep working exactly as before until they are actually removed in a later release.

Keeping the wording in one place means the install and update tasks can't drift, and gives a single home to escalate a notice (or drop it) as a deprecation moves through its lifecycle.