[ci skip] Remove guide docs for rackup

[Earlopain]

Rackup now recommends to not use it: https://github.com/rack/rackup?tab=readme-ov-file#soft-deprecation

I'd wager a guess and say that most apps (in a rails context) are already using either bin/rails server or just invoke their chosen server directly anyways.

Additionally, this info outdated. config.ru is already generated in such a way that users don't have to change anything: https://github.com/rails/rails/blob/32dcdcaef666e0043b0df844aa167201d50f8c9c/railties/lib/rails/generators/rails/app/templates/config.ru.tt

[p8]

I think this change makes sense.
I'm wondering if we can remove rackup as a dependency of Railties as well? (not in this PR)

rails/railties/railties.gemspec

Line 43 in 32dcdca

s.add_dependency "rackup", ">= 1.0.0"

Edit: I guess not as ActionPack still supports Rack 2:

rails/actionpack/actionpack.gemspec

Line 39 in 32dcdca

s.add_dependency "rack", ">= 2.2.4"

[Earlopain]

It's not so easy. Even with Rack 3 only, there is a dependency here:

rails/railties/lib/rails/commands/server/server_command.rb

Line 11 in 32dcdca

class Server < Rackup::Server

You would need to inline a bunch from this class: https://github.com/rack/rackup/blob/8d32c964c3ab7495b6fc59f30d83ae3d80bb4851/lib/rackup/server.rb

Rails does its own argument parsing, handles middleware by itself, does not expose profiling like rackup does, and probably more. I'm not sure how much relevant code would actually be left in the end (doesn't look too bad honestly) but it's definitly more involved and I'm sure there's need for deprecation somewhere as well.

[p8]

That Rackup::Server is referencing this:
https://github.com/rails/rails/blob/32dcdcaef666e0043b0df844aa167201d50f8c9c/railties/lib/rails/rackup/server.rb
See: #47885
It's basically a fallback for when rackup gem is missing.

[p8]

Also related "Require Rack 3": #50565

[Earlopain]

Rack::Server is gone in Rack 3 since it was moved to the gem. I don't quite get under which circumstances rackup/server would not be present (since railties depends on it) but it seems to definitely be required with Rack 3 at least. For Rack 2, I guess it could end up in the rescue block.

Still, anything extra to this PR will indeed be easier when Rack 2 is dropped.