Add rackup instead of rack

[andrcuns]

For the most part it's a duplicate of https://github.com/arturictus/sidekiq_alive/pull/75

There is a comment which describes why this hasn't been updated. Closing this.

[rus-max]

After upgrade rack to 3.0.8

Failures:

  1) SidekiqAlive::Server#run! runs the handler with sidekiq_alive logger, host and no access logs
     Failure/Error: before { allow(Rack::Handler).to(receive(:get).with("webrick").and_return(fake_webrick)) }

     LoadError:
       cannot load such file -- rack/handler
     # ./spec/server_spec.rb:14:in `block (3 levels) in <top (required)>'

Because Rack::Server, Rack::Handler were removed in new release to separate gem rackup and are deprecated for use. @andrcuns

[andrcuns]

@rus-max I know, and here is a comment on why we can't upgrade it yet: https://github.com/arturictus/sidekiq_alive/pull/75#issuecomment-1363308683

[rus-max]

You can release a new version with rackup and it will not affect rails applications in any way. This will be provided by rack dependencies

[andrcuns]

It will. actionpack has a version restriction set to ~> 2.0, >= 2.2.4 for rack and rackup requires rack to be >= 3. This will create a conflict.