Calling `ar_class.default_timezone` on Rails 7.0.x prints

  DEPRECATION WARNING: ActiveRecord::Base.default_timezone is
  deprecated and will be removed in Rails 7.1. Use
  ActiveRecord.default_timezone instead.

even though both accessors return the same value. The per-class
accessor was introduced as a deprecated proxy in Rails 7.0
(rails/rails@c3f43075a3) and removed entirely in Rails 7.1
(rails/rails@96c9db1b48), so the warning only fires on Rails 7.0.x.

Switch the order in PLSQL::Schema#default_timezone so that we ask the
module-level ActiveRecord.default_timezone first when it is available
(AR 7.0+) and only fall back to the per-class accessor on pre-7.0 AR
where the module-level one does not exist. This silences the warning
on Rails 7.0.x and is a no-op on Rails 7.1+ (where the per-class
accessor was already gone) and on Rails <= 6.x (where the module-level
accessor did not yet exist).

Add a regression test that captures deprecator output while resolving
plsql.default_timezone with `activerecord_class` set, and asserts that
no warning mentioning default_timezone was raised. The existing
ActiveRecord.default_timezone is captured before assignment and
restored from the ensure block alongside the deprecator behavior so
the test is order-independent.

Verified by rsim#269 / rsim#271 that the test fails on AR 7.0 without this fix
and passes on AR 7.1+ regardless.

Fixes rsim#234

References:
- rails/rails@c3f43075a3 (added the deprecated delegators in Rails 7.0)
- rails/rails@96c9db1b48 (removed them in Rails 7.1)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>