Bug 2371181

I am soon going to update rubygem-rack to Rack 3.x+ due to update of Ruby on Rails 8. Here is the PR:

https://src.fedoraproject.org/rpms/rubygem-rack/pull-request/5

But this is going to break rubygem-yard. The test suite fails with errors such as:

~~~
  1) YARD::CLI::Server General options binds address with Rack adapter
     Failure/Error: RackServer = Rack::Server

     NameError:
       uninitialized constant Rack::Server
     # ./lib/yard/server/rack_adapter.rb:12:in '<module:Server>'
     # ./lib/yard/server/rack_adapter.rb:4:in '<module:YARD>'
     # ./lib/yard/server/rack_adapter.rb:3:in '<top (required)>'
     # ./lib/yard/cli/server.rb:206:in 'block in YARD::CLI::Server#optparse'
     # ./lib/yard/cli/command.rb:56:in 'YARD::CLI::Command#parse_options'
     # ./lib/yard/cli/server.rb:223:in 'YARD::CLI::Server#optparse'
     # ./lib/yard/cli/server.rb:46:in 'YARD::CLI::Server#run'
     # ./spec/cli/server_spec.rb:64:in 'RSpec::ExampleGroups::YARDCLIServer#run'
     # ./spec/cli/server_spec.rb:202:in 'block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:129:in 'block (2 levels) in <top (required)>'
     # ------------------
     # --- Caused by: ---
     # LoadError:
     #   cannot load such file -- rackup
     #   ./lib/yard/server/rack_adapter.rb:6:in '<module:Server>'
~~~

This is due to `rackup` being split into independent package. This is going to be included in Fedora as part of bug 2338474. But I think that to prevent this issue adding `BR: %{_bindir}/rackup` could be good, since this file is already shipped by rubygem-rack

Reproducible: Always