ci: drop EOL Ruby and Rails from the matrix

[Fivell]

Drops the end-of-life Ruby and Rails legs from CI and raises the gemspec Ruby floor to match what is actually tested.

Matrix

	Before	After
ruby	['3.2', '3.4']	['3.3', '3.4', '4.0']
rails	['7.2.0', '8.0.0']	['8.0.0', '8.1.0']
activeadmin	['3.5.0']	['3.5.0'] (unchanged)

4 legs before, 6 after.

Why each removal

Per endoflife.date/ruby and endoflife.date/rails:

Removed	EOL
Ruby 3.2	2026-03-31
Rails 7.2	2026-08-09

And what stays:

Kept	EOL
Ruby 3.3	2027-03-31 (security-only — the new floor)
Ruby 3.4	2028-03-31
Ruby 4.0	2029-03-31
Rails 8.0	2026-11-07
Rails 8.1	2027-10-10

Rails 8.0 goes EOL on 2026-11-07 — about five weeks out. It is kept for now as the oldest
still-supported release, but 8.1 is the only long-lived leg in this matrix, which is why it had to
be added rather than just swapped in later.

Rails 8.2 is not released (main is 8.2.0.alpha, no tag or pre-release on RubyGems), so it is not
added.

No exclude: rules were added. In particular Ruby 3.3 x Rails 8.1 is a good cell: the
anonymous-parameter-forwarding-in-a-block SyntaxError that affects actionview 8.1 exists only in
Ruby 3.3.0, the first 3.3 release, and is fixed in later patches. ruby/setup-ruby with
ruby-version: '3.3' installs the newest 3.3 patch, so CI never sees it — and it is verified green
locally on 3.3.12 below.

The ActiveAdmin dimension is left exactly as it was; adding an AA 4.0 line is out of scope here.

Also in this PR

• tasks/test.rake generates the sample app with --skip-javascript. Rails 8.1's app generator
  writes stale_when_importmap_changes into ApplicationController, and rails_template.rb
  deliberately discards the generated Gemfile so the app runs under this gem's own sprockets bundle
  with no importmap-rails. Without the flag, every Rails 8.1 leg dies before the first example:

  Failure/Error: stale_when_importmap_changes
  NameError:
    undefined local variable or method 'stale_when_importmap_changes' for class ApplicationController
  0 examples, 0 failures, 1 error occurred outside of examples
  

  Fixed at the generator rather than by patching the generated file afterwards.

• required_ruby_version → ">= 3.3". ">= 3.0" claimed support for three Rubies that are all
  out of support and none of which is tested.

• README no longer advertises Rails 7.2 as a tested target.

The activeadmin runtime dependency (>= 3.5, < 4) is untouched — narrowing the tested range is not
evidence the gem broke on older Active Admin, and raising a runtime floor is a breaking change for
consumers. The gem version is not bumped and nothing is released.

The separate "Generate the sample app" CI step is kept as-is, so a generation failure still is not
reported as a failure of the first example.

Local verification

All runs with CI=true (the dummy app's config/environments/test.rb carries
config.eager_load = ENV["CI"].present?, and ActiveAdmin needs eager loading before the specs call
ActiveAdmin.register).

Primary leg — Ruby 3.4.10 / Rails 8.1.0 / AA 3.5.0 (resolved Rails 8.1.4):

$ CI=true RBENV_VERSION=3.4.10 RAILS=8.1.0 AA=3.5.0 rbenv exec bundle install
Bundle complete! 11 Gemfile dependencies, 107 gems now installed.

$ CI=true RBENV_VERSION=3.4.10 RAILS=8.1.0 AA=3.5.0 rbenv exec bundle exec rake setup
setup exit=0

$ CI=true RBENV_VERSION=3.4.10 RAILS=8.1.0 AA=3.5.0 rbenv exec bundle exec rspec spec
...........................................

Finished in 2.95 seconds (files took 1.18 seconds to load)
43 examples, 0 failures

New floor — Ruby 3.3.12 / Rails 8.0.0 / AA 3.5.0 (resolved Rails 8.0.5.1):

$ rbenv exec ruby -v
ruby 3.3.12 (2026-07-16 revision 0581089df9) [arm64-darwin24]

...........................................

Finished in 5.9 seconds (files took 1.94 seconds to load)
43 examples, 0 failures

Ceiling — Ruby 4.0.6 / Rails 8.1.0 / AA 3.5.0:

$ rbenv exec ruby -v
ruby 4.0.6 (2026-07-14 revision 03b6d3f889) +PRISM [arm64-darwin24]

...........................................

Finished in 3.42 seconds (files took 1.6 seconds to load)
43 examples, 0 failures

Three legs green, 43 examples each, bundle install and rake setup exit 0 on all three. The 8.0
leg generated spec/rails/rails-8.0.5.1 and the 8.1 leg spec/rails/rails-8.1.4, confirming the
Gemfile's three-segment ~> 8.0.0 pin really tests Rails 8.0 and does not drift up to 8.1.