Redmine

I have a decade-old Redmine setup using Nginx and Thin, with a configuration originally based on this howto (although I've made various changes to it over time.

I need to upgrade my web server, so I thought I would take the opportunity to also upgrade to Redmine v6. After upgrading Ruby and Rails to compatible versions and going through the upgrade process, Redmine fails with the following error:

/thin-1.6.2/lib/thin/server.rb:107:in `block in initialize': uninitialized constant Thin::Server::Fixnum (NameError)

Thin 1.6.2 is an old version, and I think Fixnum doesn't exist any more in Ruby, so I updated the Gemfile to specify the current version of thin:

gem 'thin', '>= 1.8.2'

Unfortunately...

Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Could not find compatible versions

Because thin >= 1.7.0 depends on rack >= 1, < 3
  and Gemfile depends on thin >= 1.8.2,
  rack >= 1, < 3 is required.
So, because Gemfile depends on rack >= 3.1.3,
  version solving has failed.

So if thin isn't compatible with rack v3 and Redmine v6 requires rack v3, should I abandon thin and start over with a different web server? Or is there some workaround that more experienced Ruby users can point me to?

Thanks for any help you can give.