chore: relax jruby-rack requirement to allow compatibility with upcoming 1.3.x

[chadlwilson]

There are no known changes needed to be made within warbler to be compatible with upcoming jruby-rack 1.3.x (current master/HEAD). It already doesn't use any of the removed deprecations.

Users themselves who have custom jruby-rack config via config.webxml may need to change that config, however that isn't something abstracted by warbler and they can constrain jruby-rack to a lower version if needed.

I've tested warbler locally with jruby-rack 1.3.x:

• with and without Rack 3.x support (experimental WIP) Rack 3.x support jruby-rack#325
• tested all the jruby-rack examples with warbler 2.1.1
• tested warbler with bundle exec rake spec
• tested warbler with bundle exec rake integration pointing to 1.3.x jruby-rack jar
  • Rails 7.2 test with Rack 2.2
  • Rails 7.2 test with Rack 3.1

Making this change now should allow folks to upgrade jruby-rack without another warbler release needed since it seems compatible as-is.