Consider making a new minor (or major release) with `only_hack_old_rustc` on by default

[Aaron1011]

Currently, only_hack_old_rustc must be manually enabled at the definition site. However, any crates that add a dependency on proc-macro-hack (because they want to support older versions of rustc) probably want this option on by default, as it avoids lots of unnecessary expansion and span modification when possible.

I think it would be useful to have a new minor (or major) release that enables only_hack_old_rustc by default. In addition to giving new consumers what they most likely want, it will help to nudge existing consumers to the new behavior. If any crates need the specific hygiene behavior introduced by this crate, we could have a never_hack_old_rustc option to restore the current default behavior.