Add support for rubygems 3.5+

[cboos]

'rubygems/indexer' is now shipped separately.

See rubygems/rubygems#7085.

---

When rubygems 3.5 is installed (3.5.3 shipping with Ruby 3.3), the proposed change is enough to make gem inabox work again and avoid the following error:

> gem inabox
<internal:C:/Dev/Ruby33-x64/lib/ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:127:in `require': cannot load such file -- rubygems/indexer (LoadError)
        from <internal:C:/Dev/Ruby33-x64/lib/ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:127:in `require'
        from C:/Dev/Ruby33-x64/lib/ruby/gems/3.3.0/gems/geminabox-2.1.0/lib/geminabox.rb:8:in `<top (required)>'

Besides, installing rubygems-generate_index when a version of rubygems older than 3.5 is installed seems to be harmless, but I only did limited testing (Ruby 3.0.3, rubygems 3.4.7 and gem inabox used as a client, so it likely won't use the indexer itself).

[dskecse]

Bumped into the same issue with Ruby 3.3.0 and RubyGems 3.5.4:

<internal:/Users/dskecse/.rubies/ruby-3.3.0/lib/ruby/site_ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:127:in `require': cannot load such file -- rubygems/indexer (LoadError)
        from <internal:/Users/dskecse/.rubies/ruby-3.3.0/lib/ruby/site_ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:127:in `require'
        from /Users/dskecse/.gem/ruby/3.3.0/gems/geminabox-2.1.0/lib/geminabox.rb:8:in `<top (required)>'
        from <internal:/Users/dskecse/.rubies/ruby-3.3.0/lib/ruby/site_ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:127:in `require'
        from <internal:/Users/dskecse/.rubies/ruby-3.3.0/lib/ruby/site_ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:127:in `require'
        from /Users/dskecse/.gem/ruby/3.3.0/gems/geminabox-2.1.0/lib/geminabox_client.rb:5:in `<top (required)>'
        from <internal:/Users/dskecse/.rubies/ruby-3.3.0/lib/ruby/site_ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:127:in `require'
        from <internal:/Users/dskecse/.rubies/ruby-3.3.0/lib/ruby/site_ruby/3.3.0/rubygems/core_ext/kernel_require.rb>:127:in `require'
        from /Users/dskecse/.gem/ruby/3.3.0/gems/geminabox-2.1.0/lib/rubygems/commands/inabox_command.rb:40:in `last_minute_requires!'
        from /Users/dskecse/.gem/ruby/3.3.0/gems/geminabox-2.1.0/lib/rubygems/commands/inabox_command.rb:44:in `execute'
        from /Users/dskecse/.rubies/ruby-3.3.0/lib/ruby/site_ruby/3.3.0/rubygems/command.rb:326:in `invoke_with_build_args'
        from /Users/dskecse/.rubies/ruby-3.3.0/lib/ruby/site_ruby/3.3.0/rubygems/command_manager.rb:254:in `invoke_command'
        from /Users/dskecse/.rubies/ruby-3.3.0/lib/ruby/site_ruby/3.3.0/rubygems/command_manager.rb:193:in `process_args'
        from /Users/dskecse/.rubies/ruby-3.3.0/lib/ruby/site_ruby/3.3.0/rubygems/command_manager.rb:151:in `run'
        from /Users/dskecse/.rubies/ruby-3.3.0/lib/ruby/site_ruby/3.3.0/rubygems/gem_runner.rb:56:in `run'
        from /Users/dskecse/.rubies/ruby-3.3.0/bin/gem:12:in `<main>'

/cc @tnir

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[cboos]

Ping? Any chance this could get merged?

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[zarqman]

bump