Release 8.0.0.

Major bump because the minimum Ruby is raised and EOL runtimes are dropped.

Breaking changes

• Require Ruby >= 3.3 — Ruby 3.1/3.2 are EOL and no longer supported (CI: drop EOL Ruby 3.2 / Rails 7.1, cover Ruby 4.0 and Rails 8.1 #221).
• Drop EOL Rails 7.1 from the supported/tested matrix (Rails 7.2 / 8.0 / 8.1 are tested) (CI: drop EOL Ruby 3.2 / Rails 7.1, cover Ruby 4.0 and Rails 8.1 #221).

Features

• Update existing records by id in a single import via :on_duplicate_key_update — upsert without a delete_all workaround (Ability to update or create based on id column being present #187, Support updating records by id via :on_duplicate_key_update (#187) #220).

Fixes

• batch_slice_columns now composes when called more than once: successive calls progressively narrow the column set instead of corrupting values (Calling batch_slice_columns multiple times results in unexpected behavior #186, fix: compose successive batch_slice_columns calls (#186) #223).
• A blank CSV column header now raises a clear ActiveAdminImport::Exception (blank column header at column N) instead of crashing with undefined method 'underscore' for nil (Error if csv file consists empty columns #197, fix: raise a clear error on blank CSV column headers (#197) #224).

Internal / docs

• CI now covers Ruby 4.0 and Rails 8.1; the AA 3 test app is generated with --skip-javascript to stay compatible with Rails 8.1 (CI: drop EOL Ruby 3.2 / Rails 7.1, cover Ruby 4.0 and Rails 8.1 #221).
• README: documented the two id-update strategies (native upsert vs delete-then-insert) (docs: split id-update into upsert vs delete-then-insert strategies #222).

This PR only bumps the version to 8.0.0. Tag v8.0.0, the GitHub release and the RubyGems publish follow once merged.