Add rack-up to website Gemfile for client dev

[coalest]

rack-up split off into it's own gem some time ago, so attempting to run client_dev was failing for me with a command not found error:

/.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/rubygems_integration.rb:265:in `block in replace_bin_path': can't find executable rackup for gem rackup. rackup is not currently included in the bundle, perhaps you meant to add it to your Gemfile? (Gem::Exception)
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/rubygems_integration.rb:293:in `block in replace_bin_path'
        from /.rbenv/versions/3.3.3/bin/rackup:25:in `<top (required)>'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/cli/exec.rb:58:in `load'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/cli/exec.rb:58:in `kernel_load'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/cli/exec.rb:23:in `run'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/cli.rb:455:in `exec'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/vendor/thor/lib/thor/command.rb:28:in `run'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/vendor/thor/lib/thor.rb:527:in `dispatch'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/cli.rb:35:in `dispatch'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/vendor/thor/lib/thor/base.rb:584:in `start'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/cli.rb:29:in `start'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/exe/bundle:28:in `block in <top (required)>'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/lib/bundler/friendly_errors.rb:117:in `with_friendly_errors'
        from /.rbenv/versions/3.3.3/lib/ruby/gems/3.3.0/gems/bundler-2.5.16/exe/bundle:20:in `<top (required)>'
        from /.rbenv/versions/3.3.3/bin/bundle:25:in `load'
        from /.rbenv/versions/3.3.3/bin/bundle:25:in `<main>'