Adds to the untagged 0.13.3 release.

The gemspec required Rails 8.0 from the first release commit, so applications on
the two prior Rails lines could not add the gem. Measuring the suite against
those lines shows the floor was higher than the code needs.

What the measurement showed

Full suite, SQLite, with the gemspec floor relaxed and no other change:

The single failure was the same one in all three passing runs, and it asserts a
Rails 8 error class rather than a behaviour:

EnqueueLockRetryTest#test_a_permanently_locked_database_still_raises_rather_than_hanging
[ActiveRecord::StatementTimeout] exception expected, not
Class: <ActiveRecord::StatementInvalid>
Message: <"SQLite3::BusyException: database is locked">

Rails 8 maps an exhausted SQLite busy handler to StatementTimeout. Rails 7.1
and 7.2 leave it as the StatementInvalid that StatementTimeout subclasses.
The library's own retry predicate walks the cause chain for
BusyException|BusyError, so it never depended on the Rails 8 class. The
assertion moves to the parent class and keeps the /database is locked/ match,
which is what the caller actually relies on.

Rails 7.0 is out of range

Its SQLite adapter requires sqlite3 ~> 1.4:

Error loading the 'sqlite3' Active Record adapter. Missing a gem it depends on?
can't activate sqlite3 (~> 1.4), already activated sqlite3-2.9.6

The busy-handler control this gem depends on (busy_handler_timeout=) arrived
in sqlite3 2.x, so the two cannot be loaded together. Supporting 7.0 would
mean giving up SQLite, which the gem advertises.

Change

• Gem dependencies drop from >= 8.0 to >= 7.1 for actioncable,
  actionpack, actionview, activerecord, activesupport, and railties.
  rack >= 3.1 is unchanged and still resolves on 7.1.
• The bundled migrations declare ActiveRecord::Migration[7.1]. [8.0] raises
  Unknown migration version on 7.x. Under Rails 8 the compatibility layer
  between [7.1] and [8.0] only changes remove_foreign_key, and no Solid
  Objects migration calls it, so an install built at [7.1] gets the same
  schema as one built at [8.0].
• The compatibility CI matrix runs Ruby 3.3, 3.4, and 4.0 against Rails 7.1,
  7.2, 8.0, and 8.1.
• README, docs/development.md, AGENTS.md, docs/roadmap.md, and
  CHANGELOG.md record the new floor.

Limitation recorded

The compatibility job runs SQLite only. The PostgreSQL and MySQL jobs run on the
newest Rails, so adapter behaviour on Rails 7.1 and 7.2 is unmeasured against
those servers. docs/roadmap.md states this rather than implying full coverage.

Validation

bundle exec rake on Rails 8.1.3.1 / Ruby 4.0.5: 535 runs, 1768 assertions,
0 failures, 0 errors, 15 skips. Standard Ruby, RuboCop, RBS, Steep, and Brakeman
all clean. The skip count matches main.