Deprecate add_startup_system/s

[SpecificProtagonist]

Objective

Startup systems used to require add_startup_system and belonged to special startup stages. This is no longer necessary: They can just be added to the CoreSchedule::Startup instead (and add_startup_system/add_startup_systems are one-liners doing that).

Outside of example apps, startup system aren't as useful anyways as using states is more common, so removing their special status makes sense.

Changelog

• deprecated App::add_startup_system/add_startup_systems

Migration Guide

• replace add_startup_system(my_system) with add_system_to_schedule(CoreSchedule::Startup, my_system)

[joseph-gio]

Can you update this to use .on_startup(), as added in #7790?

Also, I agree with deprecating add_startup_system, but I think we should consider leaving add_startup_systems.

I think that this:

app.add_startup_systems((
    system_a,
    system_b,
    system_c,
))

Is much more ergonomic than this:

app.add_systems(
    (
        system_a,
        system_b,
        system_c,
    )
        .on_startup(),
)

[cart]

As a heads up, we've been discussing the on_x pattern a bit on Discord (and now in github). I don't want to deprecate add_startup_system until that conversation comes to a conclusion:
#8005

[hymm]

add_startup_system and add_startup_systems were deprecated in #8079