Update CI matrix

[byroot]

Rails main is no longer compatible with Ruby 3.1.

[byroot]

There's one failure left but I don't know enough about this project to fix it:

 Error:
InstallerTest#test_installer_task:
Errno::ENOENT: No such file or directory @ rb_sysopen - app/assets/config/manifest.js
    test/installer_test.rb:19:in `read'
    test/installer_test.rb:19:in `block (2 levels) in <class:InstallerTest>'
    test/installer_test.rb:69:in `block (2 levels) in with_new_rails_app'
    test/installer_test.rb:57:in `chdir'
    test/installer_test.rb:57:in `block in with_new_rails_app'
    /opt/hostedtoolcache/Ruby/3.2.5/x64/lib/ruby/3.2.0/tmpdir.rb:94:in `mktmpdir'
    test/installer_test.rb:52:in `with_new_rails_app'
    test/installer_test.rb:8:in `block in <class:InstallerTest>'

bin/rails test /home/runner/work/importmap-rails/importmap-rails/test/installer_test.rb:7

But the CI is now way more green than it was so I'll merge.