Deprecate User::get
Deprecate User::get

The function no longer fit's a short-lived period of serenity's design,
where it would have been preferred to do `User::get(ID)` over
`UserId(ID).get()`.

Flat/serenity,zeyla/serenity.rs,acdenisSK/serenity,Lakelezz/serenity,Roughsketch/serenity,zeyla/serenity